One reason for saying "well" at the beginning of a sentence is to show that you're unsure about something.

This can happen when someone asks you a question that you don't immediately know the answer to:

A: What are you doing this weekend?

B: Well, let's see... Oh! I'm having lunch with Gavin on Saturday.

It can also happen when you're trying to say something nice, even though you're thinking things that aren't nice.

A: What do you think of Lana?

B: Well... she's... always very punctual.

You shouldn't use "well" in this way in formal written English.

This phrase appears in these lessons: