Q1. Can I use an indefinite article before 'tea' and 'cof[f]ee' and 'butter'?
A1: Yes and no.
Ex: I'll have a tea, please. <one cup of tea>
Ex: I'll have a coffee, please. <one cup of coffee>
Ex: I'll have a butter, please. /
Drinks are served in cups, and cups are countable; a tea and a coffee are short for a cup of tea and a cup of coffee. Butter, on the other hand, comes in many forms, pat, spoon, and so on, which is why, say, a pat of butter isn't usually reduced to this phrase, a butter. But some speakers might use the reduced form in casual speech; e.g., Pass me a butter (meaning, one pat of butter).
Q2. Can I use a definite article before 'butter' when I mention it for the second time around?
Ex.: A (?) butter is yellow. The (?) butter is gone bad.
A2: Yes, as long as it's defined.
Ex: The butter (that I am showing you) has gone bad.
Q3. What is a clause? I guess that it is everything that includes a verb and everything after [it].
A3: Don't forget the subject. A clause has two main parts, a subject and its verb.
Does that help?
Student or Learner