I am learning conjugated verbs, but there are no exercises to strengthen the infinitive forms of irregular verbs. At this point, it's a little bit of trouble trying to translate without having learned the infinitive first. It would be great if there were exercises to strengthen all learned variations of verbs including the original word.
There are no irregular forms of the infinitive AFAIK, they all end in either ar er (or) ir only the conjugations change. However there should be a skill for using infinitives. In Spanish there are two coming after the third checkpoint. Maybe you haven't reached them yet.
PS there are some very good grammar lessons at http://www.studyspanish.com/tutorial.htm
I see your problem now, I would recommend a good dictionary which takes a conjugated verb and gives you the infinitive. There are so many and Duo will only give a small selection of irregular conjugations within the 1700-2000 words in your tree.
My favourite dictionary is an app on my iphone from Vidalingua. There is a free version but if you want more you have to buy a few extras but they cost less than a packet of cigarettes. Alternatively a good free on-line dictionary is the answer.