Learn Hardcore Portuguese (Portugal): Os trovões assustam sempre o meu cão.

Portuguese often uses the definite article to talk about something in general. So os trovões means "thunder in general." If you drop os, it sounds more indefinite or poetic. Using os is the standard way to refer to the phenomenon of thunder as a whole.

Why is assustam in the third-person plural?

The verb assustar must agree with its subject. Here the subject is os trovões, which is plural, so you use assustam. If it were one clap of thunder, you’d say O trovão assusta..., with assusta (third-person singular).

What kind of verb is assustar? Does it need a preposition or a pronoun?

Assustar is a direct transitive verb in Portuguese, so it takes a direct object without any preposition. The one being frightened is the direct object: assustam o meu cão. You can also replace o meu cão with a direct-object pronoun (like o), yielding os trovões o assustam or enclitically os trovões assustam-no.

Why is sempre placed between the verb and the object? Can its position change?

Adverbs like sempre (always) are flexible. Placing it between verb and object (assustam sempre o meu cão) is very common. You can also say os trovões sempre assustam o meu cão (adverb before verb) or os trovões assustam o meu cão sempre (adverb at the end), though ending with sempre is less frequent. Slight emphasis shifts but the meaning stays the same.

Why is there an o before meu cão? In English we just say "my dog."

In European Portuguese, possessive adjectives (like meu, teu, seu) are normally preceded by a definite article (o, a). So you say o meu cão, a tua casa, etc. In casual Brazilian Portuguese the article is often dropped (meu cão), but in Portugal it’s standard to keep it.

Could I use a reflexive form assustam-se here?

No. A reflexive or pronominal form like assustar-se means that the subject scares itself (e.g. Ele assusta-se facilmente: "He gets scared easily"). In os trovões assustam sempre o meu cão, the dog is being frightened by an external subject (the thunder), so you need the transitive form assustar, not the reflexive assustar-se.

What’s the difference between replacing o meu cão with a pronoun versus keeping the noun?

Keeping the noun: Os trovões assustam sempre o meu cão.
Proclitic pronoun (before the verb, common when the sentence starts with the subject): Os trovões o assustam sempre.
Enclitic pronoun (after the verb, if the verb comes first): Assustam-no sempre os trovões (less common here).
All mean "Thunder always scares him/my dog," but using a pronoun makes it shorter and may require shifting word order.

Can I move sempre to modify the focus of the sentence?

Yes. If you say Sempre os trovões assustam o meu cão, you emphasize that it is always thunder that scares your dog (perhaps not other noises). If you say Os trovões assustam sempre o meu cão, you emphasize that every time thunder appears, it scares him. Subtle word-order changes can shift the focus or nuance.