Learn Hardcore Korean: gakkeum chinguwa gireul geonneoyo. - Sometimes I cross the street with my friend.

가끔 is an adverb meaning sometimes or occasionally. As an adverb, it modifies the verb and typically appears just before the verb or at the beginning of a sentence in Korean.

Why is there no subject in this sentence? Who crosses the street?

Korean often omits the subject when context makes it clear. The polite ending -요 implies that the speaker (I/저 or 나) is the one doing the action. So it means “I occasionally cross the street with a friend.”

What does 친구와 mean, and why is -와 used here?

친구와 means with a friend. The particle -와 attaches to a noun ending in a vowel to indicate “and” or “with.” Use -과 after a consonant. Here, 친구 ends in ㅜ (a vowel), so you use 와.

Could I use -하고 or -랑 instead of -와, like 친구하고 or 친구랑?

Yes. All three can mean with a friend, but they differ in formality and tone:
• 친구와 – more formal/written
• 친구하고 – neutral, everyday speech
• 친구랑 – informal, conversational