This is found in the Past Tense skill and the Tips notes have a good description of how to conjugate the verb. For present tense, you might like to consult one of the resources here, https://www.duolingo.com/comment/22040507
Here is a good source: http://moderngreekverbs.com/pigaino/ Greek does not have infinitives but rather uses the first person singular present tense. Here it's πηγαίνω.
This verb was presented in the Verb Present 1 skill Lesson 1 and the Tips & notes give an explanation of how to conjugate it in the present.
That could be and also this is a computer generated voice. But I believe in time you will become much more accustomed to spoken Greek. Try to listen several times to each sentence so the sound sinks in. That's what I did with Italian and I believe it helped me. This site has a lot of information and a good podcast by the Hellenic American Union. here