Hit the quote button.
Copy the quote tag at the end of the whole quote. Move it to the end of the sentence you want to reply to. If it's not the first sentence, copy the quote tag at the beginning of the quote, and move it to the front of the sentence. If none of the rest concerns you, delete it. If it does, then repeat the procedure above.
See if you can quote this sentence only. All you have to do is hit quote, copy the quote tag at the beginning, and paste it at the beginning of the sentence. Copy the quote tag at the end, and paste it at the end of the sentence. Delete the rest.