Learn Hardcore Finnish: Varusteet ovat tärkeitä, jos sää muuttuu nopeasti. - The gear is important if the weather changes quickly.

Varusteet is the plural form of varuste and usually translates as equipment, gear, kit.
In Finnish, many things that are a collective set in English (like equipment, clothes, supplies) are typically spoken about in the plural: varusteet, vaatteet, tarvikkeet.
So varusteet ovat tärkeitä is more like your pieces of gear are important or having proper gear is important, even though in English we would often just say equipment is important in the singular.

What exactly does varusteet mean, and how is it formed?

The base word is varuste (a piece of equipment / gear item).
The nominative plural is varusteet:

singular: varuste
plural: varusteet

Nouns ending in -e typically form the plural with -et (e.g. huone → huoneet, kirje → kirjeet).
Semantically, varusteet covers: equipment, gear, kit, outfit (for an activity), depending on context (e.g. hiking gear, safety equipment, sports gear).

Why is it ovat tärkeitä and not on tärkeä?

Varusteet is plural, so the verb must also be plural: ovat (they are), not on (it is).
In English, equipment is looks singular, but in Finnish the word is explicitly plural, so you say:

Varusteet ovat tärkeitä = The equipment / gear is important.

If you said Varuste on tärkeä, it would sound like you’re talking about one single piece of equipment (one item) being important.

Why is it tärkeitä and not tärkeät or tärkeä?

Tärkeitä is the partitive plural of tärkeä (important).
You might expect varusteet ovat tärkeät (nominative plural), and that is also grammatically possible, but there is a nuance:

Varusteet ovat tärkeät. – more definite, like these are the important ones (identifying a particular set as “the important gear”).
Varusteet ovat tärkeitä. – more general, like gear is important (in general / to some extent).

For generic statements about how something is (in general), Finnish very often uses the partitive plural, so tärkeitä sounds more natural here.

What does jos mean here, and how is it different from kun?

Jos means if and introduces something that may or may not happen:

jos sää muuttuu nopeasti = if the weather changes quickly (it might change, it might not).

Kun normally means when (in the sense of whenever / at the time that) and is used for things that are seen as factual or expected:

kun sää muuttuu nopeasti = when the weather changes quickly (implying that it does or will happen).

In this sentence we’re talking about a possible situation (conditional), so jos is the natural choice.

What form is muuttuu, and what verb does it come from?

Muuttuu is the 3rd person singular present tense of the verb muuttua (to change, to become).
Basic forms:

infinitive: muuttua = to change
he/she/it changes: hän muuttuu
the weather changes: sää muuttuu

So in the sentence, sää muuttuu nopeasti literally means the weather changes quickly or the weather is changing quickly (Finnish doesn’t mark present vs “is changing” the same way English does).

How does nopeasti relate to nopea, and why is the adverb used?

Nopea is an adjective: fast, quick (describes a noun).
Nopeasti is the corresponding adverb: quickly, fast (describes how something happens).

nopea auto = a fast car (adjective)
Auto kulkee nopeasti. = The car moves quickly. (adverb)

In the sentence sää muuttuu nopeasti, we are describing how the weather changes, so the adverb nopeasti is required, not the adjective nopea.

What exactly does sää mean, and how is it different from ilma?

Sää means weather.
Ilma literally means air, but in some contexts it can also mean weather (especially in older or more poetic style, or in certain fixed phrases).

However, for everyday modern speech, when you directly talk about weather conditions, sää is the neutral, standard choice:

Millainen sää on? = What’s the weather like?
Sää muuttuu nopeasti. = The weather changes quickly.

Using ilma here (ilma muuttuu nopeasti) would usually be understood, but it sounds less standard as a neutral “weather forecast” type sentence.

Why is there a comma before jos, and can I move the jos-clause to the beginning?

In Finnish punctuation, you typically put a comma between a main clause and a dependent clause, including clauses introduced by jos.
So Varusteet ovat tärkeitä, jos sää muuttuu nopeasti. is correctly punctuated.

You can freely move the conditional clause to the front:

Jos sää muuttuu nopeasti, varusteet ovat tärkeitä.

This version has the same basic meaning. Starting with jos slightly emphasizes the condition itself (if the weather changes quickly…) before stating the conclusion.

How do you pronounce sää and what does the ää spelling indicate?

Sää is pronounced with:

s as in English see,
ä as in the vowel of “cat”, but kept pure,
ää means a long ä sound (you hold it about twice as long as a short vowel).

So sää is roughly like säää (long ä), not two separate syllables.
The length of vowels in Finnish is meaningful: sää (weather) is different from sä (a colloquial form of sinä, you).