Hi Hassan,
“Indeed” is what I will describe as an addition to sentences, where it serves one major purpose: emphasis.
“I am grateful indeed”
Here the speaker is not just grateful, but with emphasis on his gratitude.
You can also use it to emphasise that something is true. An example:
“Is he a great teacher?
He is indeed (truly) a great teacher”
If you master those, you should be able to use the word in its different contexts. You may also construct your own sentences to reinforce your understanding.
Regards.