Learn Hardcore Finnish: Pidä sateenvarjo mukana, jos sää muuttuu nopeasti. - Keep an umbrella with you if the weather changes quickly.

Pidä is the 2nd person singular imperative of pitää (to keep/hold).
So it’s a direct command to you (one person): Keep / have (it) with you.
(For plural/polite you: Pitäkää sateenvarjo mukana…)

Why is it sateenvarjo and not some other case like sateenvarjon?

In the imperative, the object case depends on whether the action is seen as complete or ongoing/general. Here pidä X mukana is a common pattern meaning keep/have X with you (in general), so the object is typically in the partitive: Pidä sateenvarjoa mukana is also possible and may even sound more “always keep some umbrella with you.”
However, Pidä sateenvarjo mukana (nominative-looking form) is also idiomatic in everyday Finnish, especially with commands about carrying a specific item. In practice, learners will hear both; the difference is subtle and often not stressed in casual speech.

What does mukana mean, grammatically?

Mukana means with (someone), along, in one’s possession.
It’s an adverb-like form historically related to the “with” meaning (compare mukaan = along / into, more directional).
In this sentence, mukana describes the state of having the umbrella with you: keep it with you / carry it along.

Could I also say Ota sateenvarjo mukaan? What’s the difference?

Yes, and it’s very common.

Ota sateenvarjo mukaan = Take an umbrella with you (when you leave / right now) → focuses on the one-time action of taking it.
Pidä sateenvarjo mukana = Keep/have an umbrella with you (as a habit / during a period of time) → focuses on ongoing possession.

Why is jos used, and does it always mean “if”?

Jos introduces a conditional clause and usually means if: jos sää muuttuu nopeasti = if the weather changes quickly.
Finnish also has kun (“when”), which implies the speaker expects it to happen. Jos leaves it as a possibility.

Why is the subject sää in the nominative?

Because sää is the subject of the verb muuttuu (changes). Subjects are normally in the nominative.
So sää muuttuu literally = the weather changes.

What is muuttuu exactly (tense/person), and why not muuttaa?

Muuttuu is 3rd person singular present of muuttua = to change (by itself) / to become different (intransitive).
Muuttaa is a different verb pattern:

muuttaa jotakin = to change something (transitive)
muuttaa (pois) = to move (house)
So weather “changes” on its own → muuttuu, not muuttaa.

Where does nopeasti come from, and how is it used?

Nopeasti is an adverb meaning quickly/rapidly, formed from the adjective nopea (fast) + -sti.
It modifies the verb muuttuu: changes quickly.

What’s the role of the comma here?

Finnish typically uses a comma before a subordinate clause introduced by jos, kun, että, etc.
So Pidä sateenvarjo mukana, jos… follows standard punctuation rules.

Is the word order flexible? Could I put the jos clause first?

Yes. Both are natural:

Pidä sateenvarjo mukana, jos sää muuttuu nopeasti. (main clause first)
Jos sää muuttuu nopeasti, pidä sateenvarjo mukana. (condition first; slightly more emphatic on the condition)
Finnish word order is fairly flexible as long as the grammar stays intact.

How do you pronounce pidä and why is there ä?

Pidä is roughly PI-dä with stress on the first syllable (PI).
Ä is a front vowel like the a in cat (but shorter/cleaner).
Also, vowel harmony matters in Finnish: pitää has ä, so related forms usually keep front vowels: pidä, pitää, etc.