Learn Hardcore Korean: jeoneun geoui jihacheolloman danyeoyo. - I almost always get around only by subway.

나 is informal and can sound rude in polite contexts.
In many spoken sentences you can drop the subject if it’s clear, so (저는) could be omitted, giving 거의 지하철로만 다녀요.

What does 거의 mean here? Is it “mostly” or “almost”?

거의 literally means “almost” or “nearly.”

When used before a positive verb (거의 다 먹었어요 “I’ve almost eaten it all”), it means “nearly.”
With a negative verb (거의 안 가요), it can mean “hardly” or “rarely.”
In 거의 지하철로만 다녀요, it softens “only” to mean “I pretty much only go by subway.”

Why is there -로 after 지하철?

The particle -로 (or -으로 after a consonant) marks the means or method.

지하철로 = “by subway.”
It tells you how you travel.

What does -만 do, and why is it placed after -로?

-만 means “only” or “just.”

It attaches to the entire noun phrase with its particle: 지하철로 + 만 = 지하철로만.
You cannot split them or say 지하철만로.
This construction means “only by subway.”

What is the verb 다녀요, and how is it different from 가요?

다니다 means “to go regularly,” “to frequent,” or “to commute.”

다녀요 is its present polite form.
가다(가요) simply means “to go” (one‐time movement).
So 지하철로만 다녀요 implies a habitual pattern of using the subway.

Why is the sentence in present tense? Does it express habit?

Yes. The present tense here (-아요/어요 ending) plus 다니다 expresses a habitual or usual action:
“I usually get around almost exclusively by subway.”

Could I say 지하철만 타요 instead?

Yes, but the nuance shifts slightly:

지하철만 타요 = “I only ride the subway,” focusing on the single action of riding.
지하철로만 다녀요 = “I commute/travel only by subway,” emphasizing your overall pattern of movement to various places.

What’s the difference between 지하철로만 and 지하철만?

지하철로만 uses -로 (means marker) + 만, stressing that the subway is your only method of travel.
지하철만 without -로 can still mean “only the subway,” but you need a verb like 타다 (“ride”) that implies the method. With 다니다, you need -로 to show the means.