Learn Hardcore Ukrainian: Коли грім дуже сильний, собаці теж страшно. - When the thunder is very strong, the dog is also scared.

In Ukrainian, feelings like cold, hot, scary, painful are very often expressed with an impersonal construction: кому + (є) + страшно / холодно / боляче.

So:

собаці страшно literally: it is scary to the dog (to-the-dog is scary).
собаці is the dative singular of собака.

This structure focuses more on the feeling itself than on the dog as an acting subject. You could say собака боїться (the dog is afraid), but собаці страшно sounds more like a spontaneous reaction or state, very natural in this context.

What part of speech is страшно, and why isn’t it страшний to agree with грім?

Страшно here is an adverb (sometimes also called a predicative form), not an adjective. It doesn’t describe грім directly; it describes the state: it is scary.

Compare:

грім страшний – the thunder is scary (adjective страшний agrees with грім).
собаці страшно – the dog is scared / it is scary to the dog (impersonal, with страшно).

So страшно does not change for gender, number, or case; it stays the same.

Could you also say Собака боїться, коли грім дуже сильний? Is there a difference in meaning?

Yes, you can say Собака боїться, коли грім дуже сильний, and it is correct.

Nuance:

Собаці страшно... – more impersonal, focuses on the state/feeling that arises; very common in spoken Ukrainian.
Собака боїться... – uses a normal verb (боїться – is afraid), presents the dog more clearly as the subject who is doing the fearing.

Both mean the dog is afraid when thunder is strong; the original just sounds a bit more emotional and natural in this type of sentence.

Why is дуже сильний used with грім? Does it mean very loud?

Literally, дуже сильний грім is very strong thunder, but in practice that usually implies that it is very loud and intense.

Other options:

дуже гучний грім – very loud thunder (focus on the volume).
сильний грім – strong thunder (already implies loud or intense; дуже just intensifies it).

So дуже сильний is natural and idiomatic; it doesn’t sound like a mistake to a native speaker.

Could the word order be Собаці теж страшно, коли грім дуже сильний instead? Is that still correct?

Yes, Собаці теж страшно, коли грім дуже сильний is fully correct and very natural.

Word order nuance:

Коли грім дуже сильний, собаці теж страшно.
First sets the condition (when there is strong thunder), then the result.
Собаці теж страшно, коли грім дуже сильний.
First states the dog’s feeling, then explains when.

Both are fine. Ukrainian word order is relatively flexible; the comma remains in the same place because коли introduces a subordinate clause.

Why is there a comma after сильний in Коли грім дуже сильний, собаці теж страшно?

In Ukrainian, a clause introduced by коли (when) is a subordinate clause, and it is separated by a comma from the main clause.

Structure:

Subordinate clause: Коли грім дуже сильний – When the thunder is very strong
Main clause: собаці теж страшно – the dog is also scared

If you reverse the order, the comma is still needed:

Собаці теж страшно, коли грім дуже сильний.

What does теж add to the meaning? How is it different from також?

Теж means also / too and sounds a bit more colloquial and neutral in everyday speech.

In this sentence:

собаці теж страшно = the dog is scared too
(for example: children are scared, and the dog is also scared).

Також is close in meaning but often feels a little more bookish or formal.
You could say собаці також страшно, and it would still be correct, just a bit more formal in tone.

Why is грім in this form? What gender and case is it?

Грім is masculine, singular, in the nominative case.

In Коли грім дуже сильний:

грім is the grammatical subject.
дуже сильний is the predicate adjective (masculine singular nominative) that agrees with грім.

So:

masculine singular noun: грім
masculine singular nominative adjective: сильний
→ correct agreement: грім (який?) сильний.

Can грім be plural? Could you say something like коли громи дуже сильні?

Yes, грім has a plural громи, but the singular is far more common when talking about thunder in general.

You could say:

Коли громи дуже сильні, собаці теж страшно.
This is grammatically correct, but it sounds more like referring to separate claps/peals of thunder.

For a general statement about thunder during a storm, the singular грім is more idiomatic.

How do you pronounce Коли грім дуже сильний, собаці теж страшно? Where are the stresses?

Stresses (stressed syllables in CAPS):

Коли́ – ko-LY
грім – HREEM (one syllable; [iː] like in meet)
ду́же – DOO-zhe
си́льний – SYL-nyi (the first syllable stressed)
соба́ці – so-BA-tsi
теж – tezh (one syllable; zh like in measure)
стра́шно – STRA-shno (first syllable stressed)

Whole sentence (approximate):
ko-LY hreem DOO-zhe SYL-nyi, so-BA-tsi tezh STRA-shno.