Questions & Answers about Я беру ту синюю футболку, а не эту.
Why do ту, синюю, and эту all end in -у or -ю?
They are modifying футболку (T-shirt), which is the direct object of the verb беру (I am taking). Since футболка is a feminine noun in the accusative case, the demonstrative pronouns and adjectives pointing to it must also be in the feminine accusative case. The word та (that) becomes ту, синяя (blue) becomes синюю, and эта (this) becomes эту.
Why does синюю end in -юю instead of the usual -ую?
The dictionary form for blue is синий, which is a soft-stem adjective. While regular hard-stem adjectives take -ую in the feminine accusative case, soft-stem adjectives always take the ending -юю.
Why use а не instead of но не to say "not this one"?
The conjunction а is used to contrast two things directly against each other, like "that one, instead of this one." The word но is used when adding unexpected or contradicting information. Since you are simply choosing one shirt over another in a direct comparison, а is exactly what you need.
The verb беру looks very different from its infinitive. What is the dictionary form?