"I start to eat the pasta."
Translation:Je commence à manger les pâtes.
A number of verbs are constructed with preposition "à" + infinitive: aider à, apprendre à, réussir à...
Others with preposition de + infinitive: finir de, accepter de, avoir peur de...
There are dozens of rules that cover this. Counting exceptions and the need to memorize a long list of verbs that take their own form, it probably runs to hundreds.
You can approach this simply by learning the patterns of speech through repetition. Or you can take an analytical approach and try to learn the more common rules.
If your learning style is to use French, especially by speaking it with other people, then repetition is probably best for you. Otherwise, About.com is a source of seemingly endless rules, lists and variations.
If you choose About.com then try to limit yourself to those rules that are relevant to Duo's guided approach. Otherwise it's overwhelming.