Poneos doesn't seem to make sense with vosotros...

Question

Your example is:

Cuando os tumbéis en la playa poneos la crema solar.When you lie down on the beach put some suncream on.Why isn't 'poned' used here? It's an imperative command isn't it? Is it because of the 'OS' tacked on the end?

Answer

Hi Jim,I think I'm right in saying that vosotros 'poneos' that you mention derives from the reflexive verb 'ponerse'. Poneos is the imperative for vosotros. If you were using the transitive verb poner then yes the vosotros imperative form would be 'poned'.

The sentence you mention is more or less saying, "put 'yourself' in cream".   Hope this makes sense and helps a little.Clara :)

Answer

Hola Jim,
as Clara said, "poneos" derives from the reflexive verb "ponerse" and when we form the imperative for vosotros it should say: poned (imperative) + os (reflexive pronoun for vosotros), but when putting them together the "d" is omitted, so we have: "poneos" instead of "ponedos". 
Other cases for example would be:

"Niños, comeos la pizza." (Kids, eat up the pizza.)
"Lavaos las manos con jabón." (Wash your hands with soap.)
"Levantaos de las sillas." (Get off the chairs.)

There is a lesson explaining this:
El Imperative with reflexive pronouns (affirmative commands)
Saludos,
Inma