Learn Hardcore Korean: jiruhamyeon keopireul masyeo. - When you’re bored, drink coffee.

지루하다 means “to be bored.” By dropping 다 and adding the conditional ending -면, you get “if/when [you] are bored.” The subject “you” is implied and doesn’t need to be stated.

How is the informal imperative 마셔 formed from 마시다?

마시다 (to drink) drops 다 to get the stem 마시-, then you add the casual imperative ending -어 (because stems ending in 시 take -어). So 마시 + 어 contracts to 마셔, meaning “(you) drink!” in 반말 (informal speech).

Why is there no subject (like “you”) in 지루하면 커피를 마셔?

Korean frequently omits subjects when they’re obvious from context. Here, it’s clear that the listener is the one who might be bored, so you don’t need to say 너 or 당신 explicitly.

What politeness level is used here, and how can I make it more polite?

This sentence is in the lowest/informal (반말) style, used among close friends or when a speaker is clearly higher in status. To make it polite you could say:

지루하면 커피를 마셔요. (informal polite)
지루하시면 커피를 드세요. (formal polite & honorific: using -시- and -세요, and 드시다 as the honorific for “to drink”)

Can I drop the object particle 를 and say 지루하면 커피 마셔?

Yes. In fast, colloquial speech Koreans often omit particles like 를 when the meaning remains clear. In writing or formal contexts, though, it’s better to keep 를: 지루하면 커피를 마셔.

What’s the difference between 지루하다 and 심심하다? Could I say 심심하면 커피를 마셔 instead?

Both can translate as “to be bored,” but:

지루하다 describes something being dull or uninteresting.
심심하다 focuses on your personal feeling of having nothing to do.
You can say 심심하면 커피를 마셔 (“if you’re bored/have nothing to do, drink coffee”), but the nuance shifts slightly toward “I’m bored with nothing going on” rather than “this is dull.”

Is 지루하다 a verb or an adjective in Korean grammar?

In Korean grammar, 지루하다 is classified as a descriptive verb (형용사), often called an “adjective” in English. Despite the label, it conjugates exactly like action verbs (takes endings like -면, -아/어, etc.).

Are there other ways to express “if” besides -면?

Yes. The most common is -면, but you can also use:

-거든(요) (so long as; more conversational)
-다면 (more hypothetical or formal)
-(으)시다면 (adding honorific)
However, for simple real conditions, -면 is the standard choice.