Well, I will start with some basics instead of delving into the main point straight away.
There are some words in English Vocabulary that can act like a preposition as well as a conjunction.
After a preposition grammar insists that we use the objective form, for example me (Objective form of I)
Stop chasing after me.
And after a conjunction, we should have a whole clause. Basically the function of a conjunction is to connect two or more clauses, sentences and words.
You and I are from the same generation.
It was written by me and him.
Note - If you are not sure which form of pronoun (objective or subjective), please use the pronoun on its own with the conjunction removed. And you will find the correct form to be used. For example in the sentence - It was written by him and me - if you are unsure whether to use "me" or "I" after "and", please remove "and", and the sentence will come down to "It was written by me", not "It was written by I". So the correct form to use after "and" here in this case is "me".
Ours dresses are the same except mine is red. (Here the conjunction except joins a clause - mine is red)
The words - except and than - both can be used as a conjunction as well as preposition. So you have to decide whether to use objective form or the subjective form of pronoun after them accordingly.
But there is some special usage note to consider when it comes to than.
Note - Grammarians have insisted that than should be regarded as a conjunction in all its uses. So that a sentence such as Bill is taller than Tom should be construed as an elliptical version of the sentence Bill is taller than Tom is.
He is taller than I (This sentence is the elliptical version of He is taller than I am)
According to this view, the case of a pronoun following than is determined by whether the pronoun serves as the subject or object of the verb that is "understood."
But the rule allows the following sentence -
The news surprised Starun008 more than me. (It should be than me, not than I like the other example above. What is the reason? It's also simple. This concerned sentence is the elliptical version of The news surprised Starun008 more than it surprised me)
One major difference between you and me - there are no doubt many others - is that I use both cameras. (between is preposition, and so we need to use me, not I)
Neither he nor his brother can walk faster than I. (The elliptical version is Neither he nor his brother can walk faster than I can)
Whom did you mean to hurt by your unkind remarks except Sita and me? (correct. except is being used as a preposition here. So using I would be wrong)
Your husband doesn't believe that you are older than I. (The elliptical version is Your husband doesn't believe that you are older than I am)