From what I can see you need the appropriate pronoun, the appropriate version of zijn, + aan + het, and the (infinitive form of verb) to express the equivalent of I am __ing. If you want to say I am swimming, eating, drinking, etc. in English, you use the appropriate pronoun, the appropriate version of the to be verb (am, are, is), and the appropriate form of the verb (which always or almost always ends in -ing). Dutch does essentially the same thing. The main difference that you have to remember is that you use "aan het + infinitive" instead of the English verb ending in -ing (swimming, drinking, eating, etc.) Once you remember to do that it's easy! Although as yet, I haven't the faintest idea why Dutch does it this way. (Not that I know why English does a lot of the things it does, either, but back then I didn't feel as much of a need to understand how and why as I do now).