Cheers!

What's your telephone number？

It's already late.
Imagine that you are attending a party, it's getting late and you are going to leave. What do you say to the host?
for "It's already late", you can say "shi2jian1 bu4 zao3 le". "shi2jian1" is time."bu4" means no."zao3" means early."le" is a particle. It does not have any specific meaning."bu4 zao3 le", already late.
So put them together, the sentence is: "shi2jian1 bu4 zao3 le", "It's already late".

We should go now.
Next you may want to say "Wo3men gai1 zou3 le", which means we have to go. Wo3men, we. Gai1 means should, have to.zou3, to go. "le" again is a particle.Gai1zou3 le, should go.

Goodbye!
It's also such a commonly used phrase.
zai4 means again or once more. And jian4 is to see. Together "zai4jian4", similar to see you later or goodbye!

Take care.
How do you say "take care" in Chinese? you can say: duo1 bao3zhong4. Duo1 means much. bao3zhong4, "take care".
Take more care about yourself, duo1 bao3zhong4. Or you can simply say bao3zhong4.

Keep in touch.
"Keep in touch" in Chinese is "bao3chi2 lian2xi4"."bao3chi2", is to keep."lian2xi4", contact, touch or relation.