That vs Which - what is proper usage?

I'm confused when to use each and I also think that I may be inserting "that" when it is not necessary.

^For example, the above sentence is exactly what I'm unsure about. I said "I also think that I may be..." Was putting "that" in there necessary or should I have left it out so the sentence reads "I also think I may be..."

I think you shouldn't use "that". I remember my teacher telling me using "that" is more of a form of speaking than an essay and t shoud only be used if you really can't use any other way.

use of ing+verb in sentence

Although my firs language is not English, I believe your sentence is correct both with or without the word 'that'. You can use 'that', but you can leave it out (I think only when it is followed by a subject like I, you, he, she etc.).

Need clarification on using which or what??

hi,
can i use which with multiple answers?

Ex:

which are your pens among these?
or should it be :
what are your pens among these?

also

which tow children are yours among those students?
or
who are your children (two) among those students?

thanks

first one:
which one are your...

second one:
which two children are yours.

:)

thanks

but for the first one>>>

the answer is going to be "my pens are the red and the blue"

it means that the questioner knows that the answer has more than 1 choice

so can it be "which two pens are yours among these?"
or should we use "what two pens are yours among these?"

I believe that, even with your answer, it should still be "which two pens are yours among these?".

then you can write "which one is...