Jesus talks about his father a lot. Is it Yahweh? How do we know that when Jesus says, "My father" he means Yahweh from the Old Testament?
Multiple ways, though not all of them are obvious in the English translation.
Jesus refers to himself with 'I AM' in a manner as to make a direct connection to Yahweh from the Old Testament.
“Very truly I tell you,” Jesus answered, “before Abraham was born, I am!” John 8:58
God said to Moses, “I am who I am. This is what you are to say to the Israelites ‘I am has sent me to you.’” Exodus 3:14
EDIT Somebody wanted more specificity, so if you read the whole passage Jesus says clearly to the Jews...
Jesus replied, “If I glorify myself,(BN) my glory means nothing. My Father, whom you claim as your God, is the one who glorifies me. John 8:54
Doesn't get much clearer than that. The God of the Jews is Yaweh. Jesus says that the God they claim is his Father.