How to explain some words

I am a young teacher. I asked my students to write a sentence with a word: sustain.
They wrote:
We are trying to sustain our food as long as possible.
I am sustaining on a chair.
I know you feel very sad, so please let me sustain you.

and the word: evasive
Why are you so evasive. Are you scared of me?
First of all I am not sure whether I can count it good. Second: if not, what reason shall I give for that?
Thanks in advance,
Lila

Hi Lila,

Did you give them a definition first? As LL says, examples are good. Perhaps before asking them to write sentences, you could give them some form of cloze exercise, just for practice.

Why are you so evasive. Are you scared of me?

I see your problem with this sentence. You cannot tell whether the student really understands "evasive", although it is used correctly.

I think it would be reasonable to tell your student that it is a good sentence, but ask him/her to explain what they mean by "evasive". That way you can determine whether or not to count it as correct. It would be harsh not to give a mark, as the sentence is fine, but you do need to ensure they understand.

It is very important to be clear in your own mind what you want to get from your students, and to ensure they understand exactly what you want. Spend a little time planning your exercises, and if you have to spend 2 minutes concept checking to ascertain the students do understand, then so be it!

Good luck. You'll be fine.