Learn Hardcore Korean: honja isseumyeon gakkeum simsimhae. - I sometimes get bored when I’m alone.

The suffix -(으)면 attaches to a verb stem to indicate “if” or “when.” Here, 있다 → 있으면 gives “if/when (I) am).” So 혼자 있으면 literally means “if/when I’m alone.”

Why is 있다 used here? Doesn’t 있다 mean “to have”?

In Korean, 있다 has two main uses:
1) Existence (“to be” or “to exist”)
2) Possession (“to have”)
In 혼자 있으면, it’s the existence sense: “to be.” Combined with 혼자, it means “to be alone.”

What part of speech is 혼자, and why is there no particle after it?

혼자 is an adverb meaning “alone/by oneself.” When used adverbially, it doesn’t need a particle. (If you wanted to treat it as a noun you could say 혼자서 or 혼자만, but here 혼자 simply modifies 있으면.)

What role does 가끔 play, and why is it placed where it is?

가끔 is an adverb of frequency meaning “sometimes/occasionally.” It modifies the predicate 심심해, so it naturally comes before the verb. Korean adverbs are fairly flexible in placement, but [condition] + [frequency adverb] + [verb] is common.

Why is there no subject in 혼자 있으면 가끔 심심해?

Korean often omits the subject when it’s clear from context. Here the implied subject is “I” (the speaker). Dropping pronouns is perfectly normal in casual Korean.

Could I use 지루해 instead of 심심해, and what’s the difference?

Both can translate as “bored,” but:

심심하다 stresses “having nothing to do” or “feeling understimulated.”
지루하다 often describes “something” as “dull” or “tedious.”
You can say 혼자 있으면 가끔 지루해, but that puts more emphasis on your surroundings being boring rather than your own feeling of boredom.

Can I say 혼자 있을 때 가끔 심심해 instead of 혼자 있으면?

Yes. 혼자 있을 때 (using -을 때: “when…”) emphasizes a specific time when you’re alone, while -으면 can suggest a general condition or possibility (“if/when”). In everyday speech they’re often interchangeable here.