He was almost as bad at English as me.
He was almost as bad at English as I.
The first one sounds better as-is, but not when you change the second one to He was almost as bad at English as I was.
Which is correct?
To begin with, one and another are bad at English:
Using "almost as___as" to compare your proficiency, in full, one writes:
though more familiar to the ears, is wrong, as me (object pronoun) cannot take the place of he (subject pronoun) in the sentence, which is always a good way to check if these constructions are right:
If one does this switch with I, though, then it makes sense:
Errors in the usage of subject/object pronouns are quite common, especially when comparing things. In this example, the pronoun[s] used must always be the subject form. Other examples:
Using us in place of we is probably the next most popular error after I/me. Very frequently, one hears comparisons such as the following:
Though colloquial, this usage is ungrammatical. Again, one can always perform the switch in order to verify the correctness of a comparison:
It is clear that we is the correct pronoun choice.
The wrong usage of them in place of they is also another common error in this regard.
Evolved English: He was almost as bad at English as me.
Queen's English: He was almost as bad at English as I.
Redundant English: He was almost as bad at English as I was (or used to be).
That last one is equal to saying "Either he'll come tomorrow, or he won't come tomorrow."
The second is correct.
You use the form "me" when you are the object in the sentence, like in "He asked me". In the sentence above you are not the object, so the normal form "I" is used.
The "was" is implied, so including it doesn't change the meaning or the grammar. When you include it, it becomes clearer what's right.