sensei wa watashi no hyoujou o mite, 「daijoubu?」 to iimashita.

The teacher looked at my expression and said, “Are you okay?”

Breakdown of sensei wa watashi no hyoujou o mite, 「daijoubu?」 to iimashita.

• は: topic particle
• 私わたし: I
• を: direct object particle
• 先せん生せい: teacher

Questions & Answers about sensei wa watashi no hyoujou o mite, 「daijoubu?」 to iimashita.

Why is は used after 先生 instead of が?

は marks 先生 as the topic (what the sentence is “about”): As for the teacher…
が would more strongly emphasize that the teacher (and not someone else) did it, often used when introducing new information or contrasting who did the action. In a neutral narrative like this, 先生は is very common.

What does 私の表情 literally mean, and why use の here?

私の表情 means my facial expression.
の links a noun modifier to another noun, similar to ’s in English:

• 私 (I / me) + の
  • 表情 (facial expression)

What’s the role of を in 表情を見て?

を marks the direct object of the verb 見る (to see / look at).
So 表情を見て = looking at (my) expression / seeing my expression.

Why is 見て used instead of 見る?

見て is the て-form of 見る. Here it connects actions in sequence:

• (The teacher) looked at my expression, and then said…

It’s similar to English “seeing…, (they) said…” or “after looking…, (they) said…” depending on context.