Learn Hardcore Japanese: watasi no tomodati ha kenkyuusya desu.

In Japanese, の links two nouns to show possession or attribution. Here it marks 私 as the possessor of 友達, so 私の友達 literally means “my friend.”

What does は do in this sentence?

は is the topic marker. It indicates that 私の友達 is what the speaker wants to talk about. It doesn’t mark the grammatical subject in the way が does, but sets the “topic” or theme of the sentence.

What’s the difference between は and が in this context?

は (wa) marks an already-known topic or a contrast.
が (ga) marks the grammatical subject, often introducing new information or emphasizing what comes before it.
If you said 私の友達が研究者です, it would imply “It is my friend who is a researcher,” perhaps distinguishing that person from others.

What does the word 研究者 mean? Can you break down the kanji?

研究者 means “researcher.”

研究 (けんきゅう) = “research”
者 (しゃ) = “person”
Put together, 研究者 = “research person” → researcher.

What is です doing at the end of the sentence?

です is the polite copula, equivalent to the English verb “is/am/are.” It attaches to nouns and な-adjectives to make a polite statement. The plain (informal) form of です is だ.

How do you pronounce this sentence in romaji?

It’s pronounced:
Watashi no tomodachi wa kenkyū-sha desu.

Can I drop 私の and just say 友達は研究者です?

Yes. If it’s clear from context whose friend you mean, Japanese often omits known information. So 友達は研究者です simply means “(My) friend is a researcher.”

What’s the difference between 友達 and 友人?

Both mean “friend,” but:

友達 (ともだち) is more casual and common in speech.
友人 (ゆうじん) is slightly more formal or used in writing. It can sound more distant or polite.