You're very important to me, dad. Tėti, tu man esi labai svarbus. I miss you, dad. Tėti, aš pasiilgau tavęs. We use the word "tėvas" when we talk about the father as a male parent of the kids: He is a father of these children. Jis yra šių vaikų tėvas. When you talk about your father (dad), you usually use the word "tėtis": My dad is a happy person. Mano tėtis yra laimingas žmogus. I am pretty much sure there can be more explanations on these particular words and the usage of them, but that's just my two cents. Thanks for asking this kind of question because it made me think about the complexity of my own language.

Instead of "tėti" you can also use "tetuk", "tėveli"