Learn Romanian: Am pus diploma pe perete. - I put the diploma on the wall.

In Romanian, nouns immediately following a simple preposition like pe (on), în (in), or la (at) usually drop the definite article. So, even though you mean "on the wall", you just use the unarticulated form perete. You would only use the articulated form peretele if you added a description after it, like pe peretele alb (on the white wall).

Does diploma have a definite article ("the") attached to it here?

Yes! The base word for "diploma" is diplomă (ending in the special letter -ă). To say "the diploma", that final -ă changes to an -a, giving you diploma. It looks exactly like the English base word, which can be a tricky coincidence!

Is pus an irregular verb form?

Yes, pus is the irregular past participle of the verb a pune (to put). To talk about the past, you combine it with the helping verb am (I have) to make am pus (I put / I have put).

Since perete ends in -e, what gender is it?

Perete (wall) is a masculine noun. In Romanian, nouns ending in -e can be any gender, so you often have to memorize them. A good hint here is its plural form: pereți (walls), since changing the ending to -i is typical for masculine plurals.