CORRECT USE OF THAN I,THAN Me,THAN He,THAN Him

"John is taller than me" and "John is taller than I" are both correct. However, lots of your readers are likely to think that "John is taller than me" is wrong (even though it sounds natural), and a fair proportion of your readers are likely to think that "John is taller than I" sounds pretentious. The safest option is to expand the sentence after the than. For example:

"John is taller than I am."

This construction will satisfy all your readers and remove any ambiguity (more on that below).


Than I or Than Me?
Should you say "taller than me" or "taller than I"?

The quick answer is both are correct, but not everyone agrees that both are correct, and that's the problem. These days, the word than is classified both as a conjunction and as a preposition, and that's the root of the debate.

When than is used as a conjunction, it looks like this:

John is taller than I am.
or
John is taller than I.
(just a more succinct version)
However, when than is used as a preposition, it looks like this:

John is taller than me.
Grammarians have been arguing for hundreds of years over whether than is a conjunction or a preposition. Here's the bottom line. You can say:

"than I"
(than he, than she, than we, than they)
or
"than me"
(than him, than her, than us, than them)
For most people, the "than me" version sounds natural, but this is the version that runs the highest risk of being considered wrong. This is because the "than I" version has been around longer and seems more grammatically correct. The "than I" version can sound pretentious though. So, there is a lot to weigh up without any definitive guidelines.

Avoiding Ambiguity with Than Me
Unfortunately, it's not as simple as doing whatever you want and, if challenged, claiming that the world's grammarians have been squabbling for hundreds of years over this issue. There is another issue to consider: sometimes, the "than me" version introduces ambiguity. Look at this example:

John likes Peter more than me.
This could mean:

John prefers Peter to me.
or
John likes Peter more than I like Peter.
The best way to get around this is to treat than as a conjunction and write out the sentence in full. For example:

John likes Peter more than I do.
or
John likes Peter more than I like Peter.
Than Whom
There is another quirk. Nobody wants to write this:

You like him more than who?
Everyone agrees it should be:

You like him more than whom?
So, when who is the pronoun in question, than should always be treated as a preposition and you should write "than whom."
The Final Advice
The safest option is to expand the words after than. This usually means adding at least the verb (e.g., than he is / than they have). This removes all ambiguity and stops your wording sounding pretentious.

Here is an example with no potential ambiguity:

John is taller than I am.
(No one can argue this version, and it portrays you as a clear thinker.)
John is taller than me.
(This is okay, but some of your readers won't like it.)
John is taller than I.
(This is okay, but a few of your readers won't like it, and it sounds quite pretentious.)
Here is an example with potential ambiguity:

John rates Peter more than I do.
(No one can argue this version, and it portrays you as a clear thinker.)
John rates Peter more than he rates me.
(No one can argue this version, and it portrays you as a clear thinker.)