Questions & Answers about Почему ты не хочешь мерить серую футболку?
Why do "grey" and "T-shirt" end in -ую and -у here?
They are the direct object of the verb мерить (to try on), so they must be in the accusative case. The dictionary form of the noun is футболка (feminine), which changes its -а to -у (футболку). The matching adjective серая changes its feminine ending from -ая to -ую, becoming серую.
Doesn't мерить normally mean "to measure"?
It does! But in everyday Russian, мерить is also the standard word for "to try on" clothes or shoes. You are literally "measuring" the clothing against your body. You might also encounter the related perfective verb примерить used for this exact same action.
Why is the verb мерить in its infinitive form?
It comes directly after the conjugated verb хочешь (you want). Just like in English ("want to try on"), when you link two verbs together for a single action, the second one stays in the dictionary (infinitive) form.
Since the sentence uses the negative word не, shouldn't the T-shirt be in the genitive case?