You know,

"You know" is a phrase that introduces a sentence. It has lots of different uses, but one use is to make it sound softer when you have to disappoint someone or say "no" to their request:

You know, I'd love to, but I can't.

In the example above, you don't have any good recommendations for hamburger restaurants. You have to disappoint your friend, so you start with "You know" to soften it.

I'd love to (do something)

Use "I'd love to ___" when someone invites you to do something. The phrase shows that you're excited about it and you want to do it. Here are some examples:

I'd love to join you guys, if you don't mind.

A: Do you want to get together and have a drink some time?

B: I'd love to!

I'd love to, but I can't.

This is a polite and extremely common way to say "no" to an invitation. When someone invites you to do something, but you're busy at that time or don't want to do it, say this phrase and then give a reason:

I'd love to, but I can't. I'm completely broke this week.

I'd love to, but I can't. I have a karate class this evening.