3. Sorry, I can't sew these buttons UP.

Here's how we can use "up", of course.

I'm sorry, but I can't sew this sweater up. The rip is too big. :(

I also think it is important for learners to write sentences that are as complete as possible.

Therefore, I added "I'm" and "but".

We often leave words out in our casual everyday conversation. At beginner and intermediate levels students should always be encouraged to write complete sentences. The more advanced a student is, the more it is ok to present to the student how we "really use the English language" in our casual conversation. Writing complete sentences, I believe, is especially important for those learning English in a non-English speaking country. Those that are learning English in an English speaking country hear it all and therefore have some type of advantage. They can learn what is really correct from a teacher and also observe and listen to how native English speakers really use their language.

It's a tough thing to tackle, but this is how I see it.