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Grammatical tense 
is how languages talk about time

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without explicitly naming time periods

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by, instead, modifying verbs 
to specify when action occurs.

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So how many different tenses are there
in a language like English?

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At first, the answer seems obvious:

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there's past,

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present,

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and future.

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But thanks to something called
grammatical aspect,

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each of those time periods 
actually divides further.

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There are four kinds of aspect.

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In the continuous or progressive aspect,

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the actions are still happening 
at the time of reference.

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The perfect aspect describes actions
that are finished.

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The perfect progressive aspect 
is a combination,

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describing a completed part 
of a continuous action.

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And finally, there's the simple aspect,

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the basic form of the past, 
present, and future tense,

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where an action is not specified
as continuous or discrete.

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That's all a little hard to follow,
so let's see how it works in action.

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Let's say your friends tell you
they went on a secret naval mission

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to collect evidence 
of a mysterious sea creature.

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The tense sets the overall frame
of reference in the past,

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but within that, there are many options.

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Your friends might say a creature
attacked their boat,

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that's the past simple,
the most general aspect,

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which gives no further clarification.

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They were sleeping when it happened,

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a continuous process 
underway at that point.

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They might also tell you they had departed
from Nantucket

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to describe an action 
completed even earlier.

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That's an example of the past perfect.

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Or that they had been sailing 
for three weeks,

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something that was ongoing 
up until that point.

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In the present, they tell you that 
they still search for the creature today,

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their present simple activity.

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Perhaps they are preparing for their
next mission continuously as they speak.

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And they have built a special 
submarine for it, a completed achievement.

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Plus, if they have been researching
possible sightings of the creature,

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it's something they've been doing 
for a while and are still doing now

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making it present perfect progressive.

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So what does this next mission hold?

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You know it still hasn't happened
because they will depart next week,

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the future simple.

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Your friends will be searching 
for the elusive creature,

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an extended continuous undertaking.

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They tell you the submarine will have
reached uncharted depths a month from now.

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That's a confident prediction

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about what will be achieved 
by a specific point in the future,

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a point at which they 
will have been voyaging for three weeks

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in the future perfect progressive.

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The key insight to all these 
different tenses

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is that each sentence takes place
in a specific moment,

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whether it's past, present, or future.

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The point of aspects is that they tell you
as of that moment

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the status of the action.

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In total, they give us twelve 
possibilities in English.

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What about other languages?

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Some, like French,

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Swahili,

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and Russian
take a similar approach to English.

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Others describe 
and divide time differently.

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Some have fewer grammatical tenses,
like Japanese,

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which only distinguishes past 
from non-past,

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Buli and Tukang Basi,

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which only distinguish future 
from non-future,

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and Mandarin Chinese 
with no verb tenses at all, only aspect.

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On the other hand, languages like Yagwa
split past tense into multiple degrees,

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like whether something happened hours, 
weeks, or years ago.

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In others, tenses are intertwined
with moods that can convey urgency,

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necessity,

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or probability of events.

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This makes translation difficult
but not impossible.

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Speakers of most languages without certain
tenses can express the same ideas

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with auxiliary words, 
like would or did,

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or by specifying the time they mean.

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Are the variations 
from language to language

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just differents ways of describing
the same fundamental reality?

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Or do their diverse structures reflect
different ways of thinking about the world

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and even time itself?

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And if so, what other ways 
of conceiving time may be out there?