this (person/thing)

In the example above, the speaker uses "this" instead of "a". There's a small difference between the two. When you say "this ___" it sounds like you're talking about one specific thing. "A ___" is more general. 

You use "this ___" instead of "a" or "the" when you're introducing something specific that you're going to talk about. It's usually used at the beginning of a story or explanation. For example:

Amy and I were sitting there talking and this guy I know walked by.

I've had this song stuck in my head for two days!

Today I was talking with this girl at work who just got back from maternity leave.

In all of these examples, the speaker will continue by telling a story about that topic.

People only use "this ___" in this way in casual spoken English.

girl

Deciding whether to call someone a "girl" can be tricky. Here are a few guidelines:

• You usually don't call someone a "girl" if they are more than about 5 years older than you.
• A person who's old enough to be a grandmother isn't usually called a "girl".
• It's more appropriate to call a friend or someone you know socially a "girl". Calling someone you know through business a "girl" is usually not a good idea.
• Men should be more careful about calling someone a "girl" because it can seem condescending.

In the example above, the speaker is female and talking about another woman who's around the same age, so she says "girl".

at work

When you are talking about events that happened at the place where you work, you should use the phrase "at work". You say this instead of "at my job" or "at my company" when you're talking about work as a place where things happen.

Did you have a good day at work today?

(someone) who (did something)

When you want to say two things about the same person, you can combine them using "who ___". In the example above, there are two ideas that are being expressed:

I was talking with someone.

She just got back from maternity leave.

You put these ideas together by saying:

talking with someone who just got back from maternity leave

(someone) is on maternity leave

"Maternity leave" is time that you are allowed to take off of work after you have a baby. The length you are allowed to take depends on the country and your company's policies.

The word "maternity" means "being a mother". So "maternity leave" is time that a new mother takes off of work. Many companies in the U.S. also allow fathers to take time off. Because of this, the phrase "parental leave" can also be used for time taken off by either mothers or fathers. This term isn't as common, though.