Learn Hardcore Finnish: Poikkean kirjastoon ennen kuin menen kotiin.

Finnish verb endings usually show the subject, so the pronoun is often omitted.
Poikkean already means (I) drop by / detour because -n is the 1st person singular ending.
Likewise menen means (I) go.

What form is poikkean and what does the verb poiketa do here?

Poikkean is the 1st person singular present tense of poiketa.
Poiketa commonly means to drop by / to make a short detour somewhere (often briefly, not a long visit).
So poikkean kirjastoon is like I’ll pop into the library.

Why is it kirjastoon and not kirjasto or kirjastossa?

Because the sentence describes movement into the library.

kirjastoon = illative case (into): to/into the library
kirjastossa = inessive case (in/inside): in the library (location, not entering)
kirjasto (nominative) would not show the needed relationship.

Illative often answers where to? (mihin?).

How is the illative formed in kirjastoon?

kirjasto → kirjastoon uses a common illative pattern for words ending in -o/-ö:

add -on/-ön, and often the vowel lengthens: -oo-
So: kirjasto + on → kirjastoon.

Why is it kotiin (not something like kotioon)?

koti is irregular in the illative:

kotiin = (to) home (illative)
Some very common place words have fixed forms like this. Compare:
koti = home (basic form)
kotona = at home (state/location)
kotiin = (to) home (direction)

What’s the role of ennen kuin and why are there two verbs?

ennen kuin means before and it introduces a subordinate clause.
Finnish uses a full clause after kuin, so you get another verb:

main clause: Poikkean kirjastoon
subordinate clause: ennen kuin menen kotiin = before I go home

Why is menen in the present tense—does it refer to the future?

Yes. Finnish often uses the present tense for near-future plans, especially with time words like ennen kuin.
So menen kotiin can naturally mean (before) I go home even if it’s later today.

Could the word order be different?

Yes, Finnish word order is flexible and can change emphasis. For example:

Ennen kuin menen kotiin, poikkean kirjastoon. (emphasizes the timing)
Poikkean ennen kuin menen kotiin kirjastoon. (possible but can sound heavier; kirjastoon near poikkean is usually clearer)

The original is a very neutral, natural order.

How would I negate this sentence?

You negate each clause with the Finnish negative verb ei (conjugated):

En poikkea kirjastoon ennen kuin menen kotiin. = I don’t drop by the library before I go home.
If you also want to negate the second clause (different meaning):
En poikkea kirjastoon ennen kuin en mene kotiin is not a natural way to say it; you’d normally rephrase.

How do I turn it into a yes/no question?

Add the question clitic -ko/-kö to the verb (or the word you want to focus on):

Poikkeanko kirjastoon ennen kuin menen kotiin? = Am I dropping by the library before I go home?
Or focus the place:
Kirjastoonko poikkean ennen kuin menen kotiin? = Is it the library I’m dropping by before going home?

Is there a difference between poiketa and mennä here?

Yes. mennä kirjastoon is simply go to the library (neutral).
poiketa kirjastoon suggests it’s a side stop on the way somewhere else, often brief—very fitting with before I go home.

What are the key pronunciation points in this sentence?

A few important ones:

Double vowels are long: kirjastoon has long oo.
Stress is usually on the first syllable: POIK-ke-an, KIR-jas-toon, EN-nen, KU-in, ME-nen, KO-ti-in.
kotiin has long ii (even though it’s written with ii): ko-ti-in.