Learn Hardcore Finnish: Ystäväni poikaystävä sanoi, että minun pitäisi levätä useammin. - My friend's boyfriend said that I should rest more often.

Because in this structure Finnish uses the genitive form (minun) instead of the nominative (minä) to mark who is obligated to do something.

Pattern:

minun pitäisi levätä = I should rest
sinun pitäisi levätä = you should rest
hänen pitäisi levätä = he/she should rest

So the basic pattern is:

[genitive pronoun] + pitäisi + infinitive

You cannot say *minä pitäisi levätä; that is ungrammatical. The genitive (minun) is required in this necessity/obligation construction.

What exactly does pitäisi mean here? How is it different from pitää?

The verb is pitää, and pitäisi is its conditional form.

Relevant meanings of pitää:

minun pitää levätä = I have to / must rest (quite strong obligation)
minun pitäisi levätä = I should rest (softer, more polite, a recommendation)

So:

pitää (present tense) → have to / must
pitäisi (conditional) → should

In the sentence, pitäisi softens the idea from I must rest to I should rest, it would be good/appropriate if I rested.

Why is minun in the genitive case here? Is this a general rule?

Yes, this is part of a common Finnish pattern for expressing obligation or necessity.

When you have structures like:

minun pitää levätä
minun täytyy levätä
minun on levättävä
minun pitäisi levätä

the person who has the obligation is in the genitive case:

minun (I)
sinun (you singular)
hänen (he/she)
meidän (we)
teidän (you plural)
heidän (they)

So minun is genitive, required by this type of structure. It marks who is logically the subject of the levätä action, even though grammatically pitää / täytyy / pitäisi / on is the main verb.

What form is levätä? Why isn’t it lepään or lepäsin?

levätä is the basic infinitive form (dictionary form) of the verb to rest.

After pitäisi in this meaning, Finnish uses the infinitive, not a personal form:

minun pitäisi levätä = I should rest
(not *minun pitäisi lepään)

More examples of this pattern:

minun täytyy mennä = I must go
minun pitää syödä = I have to eat
hänen pitäisi lopettaa = he/she should stop

So the rule: in this structure, pitäisi + infinitive (here levätä).

Why is there a comma before että: sanoi, että?

Finnish almost always uses a comma before a subordinate clause, including että-clauses.

So:

Ystäväni poikaystävä sanoi, että…
Luulen, että hän on oikeassa. = I think that he/she is right.
Tiedän, että se on vaikeaa. = I know that it is difficult.

In English, that can often be used without a comma, but in Finnish, the comma before että is standard and expected.

What is the function of että here? Is it the same as English that?

Yes. että introduces a content clause (reported speech / what was said or thought), much like that in English.

Structure:

Main clause: Ystäväni poikaystävä sanoi = My friend’s boyfriend said
että-clause: että minun pitäisi levätä useammin = that I should rest more often

So the whole thing is:

He said that I should rest more often.

Unlike English, Finnish does not normally drop että in this type of sentence. You generally need to keep it.

Can I leave out minun and say …sanoi, että pitäisi levätä useammin?

You can, but the meaning changes.

että minun pitäisi levätä useammin = that I should rest more often
että pitäisi levätä useammin = that (one) should rest more often / people should rest more often

Without minun, it sounds more impersonal or general, like a general recommendation, not clearly about me personally.

So if you specifically mean I should rest more often, keep minun.

What does useammin mean, and how is it formed?

useammin means more often (comparative of frequency).

The pattern is:

usein = often
useammin = more often
useimmin = most often

So in the sentence:

levätä = to rest
useammin = more often

→ levätä useammin = to rest more often.

This is about frequency, not about amount of rest at one time.

What is the difference between useammin and enemmän, both sometimes translated as “more”?

They answer different “more” questions:

useammin = more often (how frequently?)
enemmän = more (how much? how many? how long?)

Examples:

Minun pitäisi levätä useammin.
= I should rest more often (increase the number of times I rest).
Minun pitäisi levätä enemmän.
= I should rest more (increase the total amount / duration of rest).

In your sentence, useammin emphasizes the frequency of resting.

Is the word order levätä useammin fixed, or can I say useammin levätä instead?

The neutral, most natural order here is:

levätä useammin

You can say useammin levätä, but it sounds more marked or slightly poetic/emphatic, like putting extra focus on useammin.

In everyday speech and writing, adverbs of frequency like usein / useammin typically come:

after the infinitive: levätä usein / levätä useammin, or
after the finite verb: lepään usein (I often rest)

So levätä useammin is the best default choice.

Could Ystäväni poikaystävä also mean my boyfriend’s friend?

No, in normal interpretation it means my friend’s boyfriend, not my boyfriend’s friend.

Breakdown:

ystäväni = my friend
poikaystävä = boyfriend

So ystäväni poikaystävä is “the boyfriend of my friend”.

If you wanted to say my boyfriend’s friend, you would reverse the order:

poikaystäväni ystävä = my boyfriend’s friend
(poikaystäväni = my boyfriend, ystävä = friend)

In these noun chains, the first word (with the possessive suffix or in genitive) is the possessor of the next.