"I am reading a book."
て form is for requests typically, but can also be used to give commands and (with いる or います) be used to say that something is currently happening.
Example: 怒る (angry) 怒っている/怒っています (am/is presently in a state of anger)
読む (will read) 読んでいる/読んでいる(am/is reading)
Can someone clarify to me what 'yonde' means? I thought it would be 'hon wo yomimasu' but I'm not sure.
読んで is the て form of 読みます. It usually denotes you are in the act of doing something. ("I am reading" versus "I read".) As a general rule, verbs that end in み, に, or び change to んで as their て form.
-te form is equivalent to 'ing'. 'Hon wo yomimasu' means 'i read a book' instead of 'am reading' so you need to conjuvate into the -te form, which uses '-nde' for verbs ending in 'mu'
When Duolingo teaches a particular tense, they want you to prove you can recognise that form by translating it into the 'same' tense in the other language.
What that means is even though you wouldn't necessarily use them in the same situations, the sentence here is using a continuous form (~ています) and they want you to use the continuous in English (-ing).
Once you get used to recognising what they're asking, you know how you're meant to answer - it's a compromise but it's how they make sure you're learning, if you're not getting it then natural translations wouldn't necessarily catch that
just to make sure I fully understand how to structure the continuous tense, it is formed by having the verb in its て/で form followed by いる/います, isn't that so?