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Here a sentence from my grammar book.
"It is as difficult to swim as drive.
I know when 'than' is used in a sentence, then it takes bare infinitive. Like..
"She is better able to speak than(bare infinitive) write."
Does it apply on my first sentence also ?
This construction has nothing to do with the equative marker as or the comparative marker than. They appear in equative and comparative constructions, where they have their own jobs. They don't control the verbs or verb forms that are used with them, so, to answer the question the way it was asked, as doesn't take anything.
The reason for the bare infinitive is that the to is not required, since the main predicate governs an infinitive in both clauses, and to was used in the first clause, so it's redundant (but possible) in the second. It's a form of conjunction reduction, though it functions inside a construction.
In the two example sentences in the question
the repeated infinitive marker to is deleted in the parallel infinitive clauses at the end, but it could have been left in. The omission is optional, not obligatory, and its optionality is part of the equative and comparative constructions. These constructions have any number of optional attachments, like a Cuisinart:
In any of the above, substitute more ... than for as ... as to get a comparative instead of an equative construction. Superlative, the highest in that syntactic paradigm, works differently from the first two, because its baseline clause doesn't have to be parallel:
insert description>.Oh, and grammar is spelled without E's. It's pronounced the same as grammer, which makes more sense, but the spelling's wrong. Normally spelling isn't noticed much, but grammar is a word you really want to spell right.
This is an example of parallelism. The full statement is "It is as difficult to swim as it is difficult to drive." "As" is functioning as a conjunction, bringing together two phrases. In both phrases, the verb is acting as the subject of the copula, with "difficult" as the subject complement (there is the dummy pronoun "it" that is, strictly speaking, directly being the subject, but the sentence is effectively "To swim is as difficult as to drive is difficult"), so the infinitive is used as it functions as a noun. The duplicated parts are not repeated, giving just "It is difficult to swim as drive". The word "to" can be included in both parts, but including "difficult" twice comes off as odd.
In your other example, the full sentence is "She is able to speak better than she is able write", with "better than" acting as a conjunction, but "better" is allowed to be moved to right after the verb, and the repeated "she is able" and "better" dropped.
Some more examples of parallelism would be "She worked longer on her Math homework than [how long she worked on her] English [homework]" and "She had found and [she had] eaten the candy" with the parts in brackets being droppable. While "She eaten the candy" is not grammatical, the parallelism allows the repeated "had" to be implied. In the same way, the "to" in your sentence is implied.
