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.

a guy

A "guy" is a man. It's a casual word.

You use this word instead of "man" when you're talking to your friends or in a casual situation like at a party. The person you're talking about can either be someone that you know, or someone who you don't know but don't have high respect for.

For a stranger who seems older and more respectable, "man" or even "gentleman" are more polite.

Note that you can use the plural form, "guys", when you're talking to a group of men or women:

Great job today, guys. Keep up the good work.

But "guy" (singular) always refers to a man.

(someone) and I

When you talk about yourself and another person, there are certain rules in English for which comes first - you or the other person:

• In casual conversation, many people say "Me and ___ (did something)"

  Me and Amywere sitting there...

  But some other people would never say this and would think it sounds uneducated.
• In polite speech and in writing, it's considered correct to say "___ and I (did something)." ("

  Amy and I were sitting there...")

• Things get confusing when you put this at the end of a sentence. A lot of people still say "(do something) to ___ and I." But some people think this is incorrect and you should say "(do something) to ___ and me." Since everyone disagrees, you can probably just pick which one you like!

  He said something to Amy and I.

  He said something to Amy and me.

sitting (somewhere) (doing something)

You can combine "sitting" or "standing" with another action in this way. A few more examples:

I was sitting on the couch doing my homework.

They're still standing outside the train station waiting for someone.

Describing an action in this way with "sitting" or "standing" makes it sound like the action wasn't very important or interesting. This kind of phrase is often used to set up a story, like in the example above.

a (person) (someone) knows

"An acquaintance" is a person you know who's not close enough to be called a "friend". But the word "acquaintance" is pretty formal. When you're telling a story or talking to a friend, it's more natural to say "a person I know", "a woman he knows", "this guy she knew", etc.:

This guy I know is from Laos, and he said that there are a lot of expats living there now.

walk by (someone)

To "walk by" someone means to walk past them. These would all count as "walking by" someone:

• Person A is standing in a hallway, and Person B walks down the hallway past Person A's position.
• Person A is sitting on a bench facing a sidewalk. Person B walks down the sidewalk toward the bench, then continues walking past the bench.
• Person A and Person B walk toward each other, then continue on past each other.

You can use "walk by in the following ways:

Yuichiro just walked by.

Brendan walked by me without saying 'hello'.