"As far as I know, we can not use the 'ing' form after "to" in English" That's not true at all, Meysam. There are actually many cases where the gerund (+ing form) can follow 'to'. In fact, the only time when 'to' is followed by the base form of the verb ( 'finish') is when the 'to' forms part of the infinitive, for example: I want to finish the job. I decided to finish the job. In other situations, 'to' acts like any other preposition - such as with, of, from and so on - and is followed by the gerund. In the case of 'I had committed to doing the job', the 'to' is part of the verb phrase 'commit to [something]', and so the next word has to be either a noun or a gerund. Here are some more common examples of phrases with 'to' that are followed by the gerund: I'm looking forward to seeing you. I'm used to getting up early. I'm addicted to watching soap operas. I confess to being a technophobe. I hope that helps.

The simplest form of the infinitive "to ___" is just to place a verb after it. Examples: "to eat" "to run" etc. However, there is the progressive verb form with "___ ing" that is also used with the infinitive "to". Usually, we find this as in the examples: "So nice to be writing to you on italki." or "You do not need to be worrying about this problem." When progressive form "ing" is used, it means that the action is continuing around the same time the speaker is talking. Your sentence above is a little different than my examples, but the speaker is stating that the job that he/she had committed to still needs to be finished. It is in a progressive state and needs to happen at a continuing time as the present.

In this sentence "finishing" is a noun, like "completion". So it's not a continuous form of the verb "finish", they just spell identically.

The other three comments have probably explained it pretty well. But I thought I would add that it might help to think of "finishing the job" as a noun.

Thank you guys