1. This book was the most helpful for my homework.

Question

2. This book was most helpful for my homework.

3. Tom is the youngest of his brothers.
4. Tom is youngest of his brothers.

5. He is the tallest in his class.
6. He is tallest in his class.

(Which one is grammatical in each pair? Do we have to use 'the' or not? Which ones are commonly used?)

Answer 1

I explained #1 earlier.

In all the others, it doesn't make much difference if you use "the" or not.

=)