Given that a gerund after "to" is considered as a spelling mistake, I'm going to need a much clearer explanation I guess.
2
Googled it for you as I'm too lazy to type this all out myself:
I take it that you are confused about when to use "to help" (or some other verb) and when to use "to helping" (or some other VERB+ing).
The way to differentiate between the two is to see if you can put a noun after the word "to" in the sentence you are writing. If you can put a noun after "to" and the sentence makes sense, then you should use a gerund (VERB+ing). This is because a gerund is VERB+ing used as a noun. If not, then you should not use the "-ing" ending. In this way, we get an infinitive (to + VERB). The difference between the two forms is that in the first case, the word "to" is a preposition, whereas in the second case, "to" is part of the infinitive (INFINITIVE = to + VERB), not a separate grammatical element in the sentence.
If we apply this test to the example sentence, we could say something like:
Mother Teresa devoted her life to service (noun).
Mother Teresa devoted her life to the poor (noun).
Clearly, we can put a noun after "to" in this sentence. We can feel confident that the place after "to" in this sentence is a place for a noun, (and remember that a gerund does the work of a noun). Since the sentence meets the test, we can feel safe in using a gerund in the place of the noun after "to." In this case, "to" is clearly a preposition (since it is followed by a noun). So we say:
Mother Teresa devoted her life to helping the poor. [preposition + gerund]
Let's look at another sentence:
Mother Teresa decided to _____________.
In this sentence, it would be impossible to put a noun in the blank after "to" and have the sentence still make sense. We cannot say: "Mother Teresa decided to service." Since this sentence does meet the test, we need to use "help" without the "-ing." This creates an infinitive (to + VERB). So we get:
Mother Teresa decided to help the poor. [infinitive]
Consider another common construction that causes confusion: "look forward to." Should there be a gerund after this or just the simple form of the verb without -ing? If we apply the test, we get sentences like this:
They are looking forward to the party. (noun)
I am looking forward to your response. (possessive pronoun + noun)
Clearly the place after the word "to" in this case is a place for a noun, and "to" is therefore a preposition. So we use a gerund.
I look forward to hearing from you. [preposition + gerund]
They are looking forward to meeting you at the party. [preposition + gerund]
But we cannot say:
I look forward to hear from you. [INCORRECT]
They are looking forward to meet you at the party. [INCORRECT]
NOTE: Infinitives can be used as nouns just as gerunds are used as nouns. However, it is important to remember that the word "to" is included in the infinitive, whereas "to" is not part of the gerund, but is a preposition that comes before the gerund as a separate grammatical element.
In the following sentences, "to help" is used as a noun, but "to" is part of the infinitive, not a separate grammatical element.
To help the poor was Mother Teresa's desire from an early age.
Mother Teresa wanted only one thing in life: to help the poor.