When someone greets me with "salaam," is it more proper to respond "salaam," or should I respond "as-salamu alaykum"?

Question

I have some Muslim friends and they use different forms of greeting, so I am a little confused.

What would be a proper response to "salaam"?
When should I use the full "as-salamu alaikum"?
When should I reply with "wa-alaikum-salaam"?

Answer 1

Saying:

Salaam

is an incomplete greeting.

And Allah the almighty taught us how to respond:

And when you are greeted with a greeting, greet [in return] with one better than it or [at least] return it [in a like manner]. Indeed, Allah is ever, over all things, an Accountant. (4:86)

so the minimum would be to return it (but that would be incomplete as gthe correct greetg is:

As-Salamu 'Alaikum

, however it is certainly better to replay:

(wa) Alaikum as-Salam
As-Salamu alaikum

and much better to say:

(Wa) 'alaikum as-Salamu wa rahmatu-(A)Llah
As-Salamu 'alaikum wa rahmatu-(A)Llah

or

(Wa) 'alaikum as-Salamu wa rahmatu-(A)Llah wa barakatuhu
As-Salamu 'alaikum wa rahmatu-(A)Llah wa barakatuhu

See also How should I reply to "Assalamu Alaikum"? and What is the meaning and the usage of "Assalamu alaikum"?