Learn Portuguese (Portugal): Tu fazes sempre isso no escuro?

Fazes is the tu (you, informal) form of fazer (to do/make). The biggest irregularity for this verb in the present tense is the eu form (eu faço). The tu form (fazes) actually looks quite normal for an -er verb, just dropping the -r and adding -s.

Why does the sentence use isso instead of esse for "that"?

Isso is an invariable word used for abstract ideas, actions, or unidentified things near the listener. Because "that" in this sentence refers to an action ("doing that") rather than a specific masculine noun like "that book", we use the neutral isso.

Why does sempre come after the verb, unlike in English?

In Portuguese, adverbs of frequency like sempre (always) typically go immediately after the verb they modify. So while in English we say "you always do", in Portuguese it is naturally ordered as tu fazes sempre (you do always).

How is no escuro formed?

No is a mandatory contraction of the preposition em (in) and the masculine definite article o (the). Just like in English, we say "in the dark", so Portuguese combines em + o to make no escuro.