Why does depuis use the present tense in French?

Because depuis describes an action that started in the past and is still going on now - and French sees that as a present-tense fact. J'apprends le francais depuis trois ans means 'I have been learning French for three years', but the verb apprends is present, because you are still learning right now. English reaches for the perfect ('have been learning'); French keeps it present because the action is ongoing. This is the single biggest trap with depuis: never put the verb in the passe compose when the action is still continuing.

What is the difference between depuis, pendant and il y a?

Depuis = 'since / for' an action still going on, with the present tense (j'habite ici depuis 2015 = I have lived here since 2015). Pendant = 'for / during' a finished, bounded stretch of time, usually with the passe compose (j'ai travaille pendant deux heures = I worked for two hours). Il y a = 'ago', marking a point in the past (il est parti il y a deux jours = he left two days ago). Depuis is ongoing, pendant is bounded, il y a is a point back in time.

French Time Expressions: depuis, pendant, il y a, pour

Four words do most of the work of expressing time in French: depuis, pendant, il y a, pour. They look similar in English glosses - "for", "since", "ago" - but they are not interchangeable. The key is what each one does to the tense of the verb.

J'habite ici depuis 2015. (I have lived here since 2015 - still living here.)
J'ai travaille pendant deux heures. (I worked for two hours - finished.)
Il est parti il y a deux jours. (He left two days ago.)
Je pars pour une semaine. (I'm leaving for a week - planned ahead.)

depuis: "since" or "for", with the present

Depuis marks an action that started in the past and is still going on. It translates as "since" (a start point) or "for" (a duration).

The trap: depuis takes the present tense where English uses the perfect ("have lived", "have been learning"). The action is still happening, and French treats that as a present fact.

J'habite ici depuis 2015. (I have lived here since 2015.)
J'apprends le francais depuis trois ans. (I have been learning French for three years.)
Elle travaille dans cette entreprise depuis janvier. (She has worked at this company since January.)
Nous attendons depuis vingt minutes. (We have been waiting for twenty minutes.)

In every one of those, the English verb is a perfect ("have lived", "have been learning") and the French verb is a plain present (habite, apprends, travaille, attendons). The action runs up to now, so French keeps it present.

Since versus for is just a question of what follows depuis:

A point in time (a year, a date, an event) = "since": depuis 2015, depuis janvier, depuis mon arrivee.
A duration (a number of years, hours, days) = "for": depuis trois ans, depuis vingt minutes.

The verb tense does not change between the two; only the English translation does.

depuis with the imperfect: "had been doing"

Shift the whole situation into the past and depuis pairs with the imparfait instead of the present. This describes something that had been going on for a stretch when something else happened.

J'attendais depuis une heure quand il est arrive. (I had been waiting for an hour when he arrived.)
Elle habitait a Paris depuis cinq ans quand elle a demenage. (She had been living in Paris for five years when she moved.)

The logic mirrors the present-tense rule. With the present, depuis means "have been doing (and still am)". With the imperfect, it means "had been doing (up to that past moment)". Either way, the verb is not a completed past tense - it's the ongoing one.

pendant: "for" or "during" a bounded duration

Pendant measures a finished, bounded stretch of time. The action has a clear start and end, and is usually reported in the passe compose.

J'ai travaille pendant deux heures. (I worked for two hours.)
Il a vecu en Espagne pendant dix ans. (He lived in Spain for ten years - and no longer does.)
Pendant les vacances, nous avons visite trois pays. (During the holidays, we visited three countries.)

The contrast with depuis is the whole point. Depuis = still going on (present). Pendant = over and done (passe compose). "J'habite ici depuis dix ans" means you still live here; "j'ai habite ici pendant dix ans" means you have since moved on.

Pendant can often be dropped before a duration with no change of meaning: j'ai travaille deux heures is as natural as j'ai travaille pendant deux heures.

il y a: "ago"

When il y a is followed by a length of time, it means "ago" - it pins an event to a point in the past, measured back from now. The verb is normally in the passe compose.

Il est parti il y a deux jours. (He left two days ago.)
J'ai fini il y a cinq minutes. (I finished five minutes ago.)
Nous avons demenage il y a un an. (We moved a year ago.)

Do not confuse this with the other il y a, which means "there is / there are" (il y a un probleme = there is a problem). It's the il y a + duration combination that means "ago".

pour: "for" a planned future duration

Pour expresses a duration that is planned or intended, usually looking forward. You set out to spend a certain amount of time.

Je pars pour une semaine. (I'm leaving for a week.)
Elle est en France pour trois mois. (She is in France for three months.)
Nous sommes ici pour le week-end. (We are here for the weekend.)

The catch for English speakers: pour is for an intended duration, not the actual elapsed one. To say how long an action actually lasted, use pendant, not pour. "J'ai travaille pour deux heures" is wrong for "I worked for two hours" - that's pendant. Save pour for the planned stretch ahead.

The four side by side

Word	English	What it marks	Typical tense	Example
depuis	since / for	action still going on now	present (or imperfect)	j'habite ici depuis 2015
pendant	for / during	a finished, bounded duration	passe compose	j'ai travaille pendant deux heures
il y a	ago	a point in the past, measured from now	passe compose	il est parti il y a deux jours
pour	for	a planned / intended future duration	present / future	je pars pour une semaine

Read the table by the middle column. Is the action still going (depuis), finished and bounded (pendant), a point back in time (il y a), or planned ahead (pour)? That choice decides the word and drags the tense along with it.

Worked examples

Je connais Marie depuis l'ecole primaire. (I have known Marie since primary school.)
Tu apprends le piano depuis combien de temps? (How long have you been learning the piano?)
Il a plu pendant toute la nuit. (It rained the whole night.)
Nous avons attendu le train pendant une heure. (We waited for the train for an hour.)
J'ai recu ton message il y a une heure. (I received your message an hour ago.)
Ils sont partis en vacances pour deux semaines. (They have gone on holiday for two weeks.)
Elle dansait depuis l'enfance quand elle a eu son accident. (She had been dancing since childhood when she had her accident.)

Common mistakes English speakers make

Translating "have lived / have been doing" with a French perfect after depuis: j'ai habite ici depuis 2015 is wrong, because the action is still going on, so it stays present - j'habite ici depuis 2015. Using pour for an actual elapsed duration: j'ai travaille pour deux heures should be pendant deux heures - pour is only for the planned future stretch. Reaching for pendant when the action continues to now: a still-running situation wants depuis, not pendant. And missing that il y a flips between "there is" and "ago" depending on whether a duration follows it.