て form is also used when you are describing what someone is doing at that moment.
So normally, you could say something like サリーさんはしんぶんをよみます。Sally reads a newspaper. So, let's break it down first.
サリーさん は しんぶん を よみます。
Miss Sally reads a newspaper.
This is the basic sentence we'd use in Japanese, but what happens if someone asks:
サリーさんはいまなにをしていますか。 What is Sally doing now?
サリーさん (Miss Sally) は (particle) いま (now) なに (what) を(particle) しています (doing) か (question)
You might think that the question will actually end in をしますか but it doesn't, the reason being is that をしますか implies future tense, which will contradict your use of the word 'now' いま. So, the question ends in をしていますか.
You need て form to describe what you are doing now, too.
The て form of よみます is よんで which we learnt in the blog post about the て form in general. Add います to よんで to finish the sentence and make it polite.
So finally, the sentence would be サリーさんはいましんぶんをよんでいます。Miss Sally is reading a newspaper now.
You can ask lots of things and give lots of answers with this! All you have to do is replace the object and the verb, and of course the person.
[Person]は [Time] [Object] を [Te form verb] います。
カイさんはいまなにをしていますか？What is Kai doing now?
カイさんはいまおちゃをのんでいます。Kai is drinking Japanese tea now. (おちゃ)