Of course, you do not have to type the text you are replying to manually, and you do not have to copy and paste it from the original message either. Yahoo! Mail Classic can automatically include it in your reply.
Quote Original Messages in Your Replies in Yahoo! Mail Classic
To have Yahoo! Mail Classic include the original message's text in your reply:
- Follow the Options link in the Yahoo! Mail Classic navigation bar.
- Select General Preferences.
- Make sure Include full original message when replying is selected under Message Actions | Replying.
- Click Save.
I suggest choosing Include full original message when replying instead of Include part of the original message when replying, because the latter makes Yahoo! Mail Classic truncate the original message after a certain number of lines. If something important is below that fold, it makes more sense to cut out another part of the original message to keep the reply short instead.