just (do something)

You use this phrase when the action you're describing seems easy, simple, fast, unimportant, or unexciting:

I just googled "new york florist" and that was the first shop that came up, so I called them.

A: What did you do this weekend?

B: I just sat at home and watched T.V.

keep track of (something)

When you keep track of something, you remember it, record it, or write it down so that you don't forget. Things that people often "keep track of" include:

• keep track of time
• keep track of your money
• keep track of your progress
• keep track of your kids
• keep track of important events

how (much/many)(someone)(does)

This is a phrase to use when you want to discuss a number as a noun:

I don't remember how many I ate.

There's some debate over how much we should spend on advertising.

pay (someone) back

To "pay someone back" means to return money that you borrowed from them. For example, when you have to borrow money from your roommate you can say:

I promise I'll pay you back by the end of the week.

Or, in situations like the one at the top of this post, it means to give someone money when they paid for an item that you both shared, like a lunch.