"Pedro usually puts on a T-shirt."
Translation:Pedro usualmente se pone una camiseta.
I could be wrong, but I think 'ponerse' is supposed to go after an already conjugated verb to express what the person is going to do and 'se pone' is what is already happening. i.e.: Pedro quiere ponerse una camiseta = Pedro wants to put on a t-shirt Pedro se pone una camiseta = Pedro puts on (or is putting on) a t-shirt.
I hope this helps! :)
There is no way that sentence was given earlier.
You need to conjugate the verb. It's third person singular. So it should be pone. In these exercises, Duo is mixing up the verbs so they are conjugated at times, and infinitives at other times. This is a great way to learn verb usage, and also how Spanish uses the reflexive pronoun. Take the opportunity to let Duo teach you, instead of arguing that Duo is wrong.
"Pedro puts" - 3rd person singular - "Pedro pone"
"Pedro wants to put" - 3rd person sing + infinitive - "Pedro quiere poner"
For the conjugated form, the reflexive pronoun goes before the verb - "se pone". For the infinitive, attach the reflexive pronoun to the end of the infinitive - "ponerse".
Yes, it can be confusing. That is why Duo is doing these exercises for you!
Think, learn, be accurate.