Could somebody explain how to use these words in sentences and what is the difference between them?
For example in sentences like that (but not only):
Joanne is the most intelligent person I know.
Joanne is most intelligent person I know (it`s incorrect I suppose)
The department needs three more computers in order to work most effectively
The department needs three more computers in order to work the most effectively
Harry's reply is a bit too concise; just in case, I'll elaborate a little.
The most is used with the superlative degree of adjectives:
Did you happen to see the most beautiful girl in the world?
With adverbs, if you have in the sentence or imply at all, you can use the, it's optional.
Who knows grammar the best of all?
In an informal style you can drop (omit) the:
Which of them is brightest?
Who can explain best?
We do not use the in the superlative, if we compare the same person or thing in different situations:
The president is nicest when he can smell some investments.
To me, coffee is most delicious with a drop of cognac.
most , however,can mean very. In this case we use the indefinite article with countables and no article with uncountables:
X. is a most devious politician.
This is most unpleasant weather.
You are most welcome.