Learn Hardcore Korean: jega jaju masineun keopiga masisseoyo. - The coffee that I often drink is delicious.

One 가 marks 저 in the relative clause (제가 마시는) to show “I” as the subject of 마시다. The second 가 marks 커피 in the main clause as the subject of 맛있다. In Korean, when you embed a clause before a noun (a relative clause), you still need its own subject marker inside that clause. Then the resulting noun phrase takes its own subject particle in the main sentence.

Why is 마시는 used to modify 커피 instead of 마시다 or 마셔요?

마시는 is the present-tense attributive form of 마시다, used to describe a noun: “the coffee that (I) drink.”

마시다 is the dictionary (infinitive) form and can’t directly modify a noun.
마셔요 is a polite sentence-ending form and also can’t modify a noun.
If you wanted past modification, you’d use 마신 (“the coffee that I drank”).

What is the function of 제가 in the relative clause, and could you use 저는 or omit it?

Inside a relative clause, only the subject marker -이/가 attaches to the noun performing the action—in this case 저 + 가 = 제가. You cannot use 저는 there because -는 marks topics in main/predicate clauses, not subjects inside modifiers. You could omit 제가 altogether if context is clear, but then the sentence would simply say “the coffee I often drink is delicious” without explicitly stating “I.”

Why is there no object particle -을/를 before 커피 in 마시는 커피?

In Korean relative clauses, the head noun being modified always comes at the end of the clause and does not carry its case marker inside that clause. Here, 커피 is both the object of 마시다 and the head noun of the modifier, so you drop -을/를. When the entire clause ends, you attach the appropriate particle to that head noun in the main clause (커피가).

Why is 자주 placed before 마시는, and can it go elsewhere?

자주 is an adverb meaning “often,” and Korean adverbs usually precede the verb (or verb form) they modify. In this sentence, it modifies the attributive verb 마시는. You could move it slightly—제가 마시는 커피를 자주 마셔요 (I drink coffee often)—but inside a relative clause you normally keep adverbs right before the verb: 제가 자주 마시는 커피.

Why do we use 커피가 instead of 커피는 in this sentence?

-가 is the subject marker used when presenting new information or describing a property neutrally (“this coffee is delicious”). -는 is the topic marker and often implies contrast or known context (“as for the coffee…”). Here, you’re simply stating that the coffee you often drink is tasty, so 커피가 is more natural.

What level of politeness is conveyed by 맛있어요, and how can I adjust it?

맛있어요 is the standard polite form (ending in -요) used in daily conversation.

More formal: 맛있습니다 (deferential/formal).
Casual: 맛있어 (used with close friends or younger people).

Could I say 제 커피 instead of 제가 자주 마시는 커피 to mean “my coffee”?

제 커피 (my coffee) uses the possessive 제 (저의 shortened) and simply states ownership. 제가 자주 마시는 커피 literally means “the coffee that I often drink,” providing extra detail about which coffee you’re talking about. They’re not interchangeable if you want to emphasize the habitual action of drinking.