Learn Hardcore Swedish: Det är inte vanligt att han dricker kaffe sent på kvällen. - It is not common that he drinks coffee late in the evening.

Det here is a dummy subject (also called an expletive). It doesn’t refer to anything specific; it just fills the subject position, like “it” in English:

Det är viktigt att du kommer. – It is important that you come.
Det är inte vanligt att han dricker kaffe sent på kvällen. – It is not common that he drinks coffee late in the evening.

So det is there for grammar, not meaning.

Why do we need att before han dricker kaffe?

Att introduces a clause that acts like a “thing” in the sentence – here, it’s the thing that is “not common”:

Det är inte vanligt [att han dricker kaffe sent på kvällen].

In English, this att is usually translated as “that”:

It is not common *that he drinks coffee late in the evening.*

You cannot omit att in standard written Swedish in this kind of clause. In fast speech, Swedes may mumble or reduce it, but it’s still there grammatically.

Could you say Han dricker inte kaffe sent på kvällen instead? Is the meaning the same?

They are close, but not identical:

Han dricker inte kaffe sent på kvällen.
= He doesn’t drink coffee late in the evening.
This states it as a fact: he doesn’t do it.
Det är inte vanligt att han dricker kaffe sent på kvällen.
= It’s not common for him to drink coffee late in the evening.
This focuses on how typical it is. It suggests he may do it sometimes, but it’s not his usual habit.

So the original sentence is softer and more about habit/frequency than about an absolute rule.

What’s the difference between inte vanligt and ovanligt?

Both can be translated as “not common” / “unusual”, but the nuance is different:

inte vanligt = not usual, not common
Neutral description; it may still happen, just not regularly.
ovanligt = unusual, rare
Stronger, more marked. Often sounds a bit more emphatic.

Compare:

Det är inte vanligt att han dricker kaffe sent på kvällen.
= It doesn’t happen often; it’s not his habit.
Det är ovanligt att han dricker kaffe sent på kvällen.
= It’s more striking/surprising when he does; it’s quite rare.

Why is the verb dricker (present tense) used if we’re talking about a habit?

In Swedish, the present tense is used for:

actions happening now
habits and general truths

That’s the same as English simple present:

Han dricker kaffe varje morgon. – He drinks coffee every morning.
Han dricker kaffe sent på kvällen. – He drinks coffee late in the evening.

So dricker here is present tense describing his general habit, not just something happening right now.

Why is kaffe used without an article? When would you say kaffet?

Kaffe is usually treated as a mass noun (like water, wine):

Han dricker kaffe. – He drinks coffee. (some coffee, in general)
Vill du ha kaffe? – Do you want (some) coffee?

You use kaffet when you mean the specific coffee already known from context:

Kaffet är kallt. – The coffee is cold.
Han dricker inte kaffet. – He doesn’t drink the coffee. (that particular coffee)

In your sentence it’s about the activity of drinking coffee in general, so no article: dricker kaffe.

Why is inte placed after är (Det är inte vanligt) and not somewhere else?

Swedish main clauses follow a verb-second pattern: the finite verb (here är) normally comes in second position. The negation inte comes after that verb:

Det är inte vanligt. – It is not common.
Han dricker inte kaffe. – He does not drink coffee.

So the order is:

Subject (or some other element in first position)
Finite verb
inte (and other sentence adverbs)

Hence: Det – är – inte – vanligt …

What does sent på kvällen literally mean, and why på and not i?

Literally:

sent = late
på kvällen = in the evening / on the evening

So sent på kvällen = late in the evening.

Swedish normally uses på with parts of the day when talking about time in general:

på morgonen – in the morning
på eftermiddagen – in the afternoon
på kvällen – in the evening
på natten – at night

You don’t say i kvällen in this sense. I kväll (without -en) is a different expression meaning tonight.

What’s the difference between sent på kvällen and på kvällarna?

sent på kvällen
= late in the evening (one evening, or “in the evenings” understood from context, but with focus on the late time of day)
på kvällarna
= in the evenings (plural, talking about evenings in general, as a repeated habit)

You can also combine both:

sent på kvällarna
= late in the evenings (repeated habit, stressing that it’s late)

What grammatical form is kvällen, and why not just kväll?

kväll = evening (indefinite form)
kvällen = the evening (definite form)

In time expressions with parts of the day, Swedish normally uses the definite form after på:

på morgonen – in the morning
på eftermiddagen – in the afternoon
på kvällen – in the evening
på natten – at night

So sent på kvällen is the standard pattern; sent på kväll would sound wrong.

Could you leave out att han and say Det är inte vanligt dricka kaffe sent på kvällen, like English “It’s not common to drink coffee late in the evening”?

Not with han included. You have two correct options, with different meanings:

Det är inte vanligt att han dricker kaffe sent på kvällen.
= It’s not common that he drinks coffee… (about this particular person)
Det är inte vanligt att dricka kaffe sent på kvällen.
= It’s not common to drink coffee late in the evening. (in general, for people)

Swedish does not allow you to just put a bare infinitive phrase here when you have a different, explicit subject (han) inside the clause. You need att plus a full clause (att han dricker …).