don't give too much focus on how the word changes from "ajar" to "mengajar", "belajar", "diajari", etc. Just use the words often, like this:
Tina mengajar Tuti (Tina teaches Tuti)
Tina mengajari Tuti (Tina teaches Tuti (more intensive))
Tuti diajar Tina (Tuti is taught by Tina)
Tuti diajari Tina (Tuti is taught by Tina (intensively))
Ajaran agama (religion teaching)
Ajari aku (teach me)
Tina belajar (Tina studies)
Tina sedang belajar (Tina is studying)
Tina mengajar bahasa Inggris (Tina teaches English)
Tina mengajarkan bahasa Inggris (Tina teaches English)
Pelajaran (lesson)
and so on...
I hope it can help..

Depends on the person. However, I see many people have difficulty to learn about Indonesian suffix, prefix, or kind of. My suggestion, don't really too much focus on the changes of the suffix, prefix, affix, and so on. Just use the words in sentences often. For example, we know the basic word is "ajar". Now, if we want to use it in a sentence, whether is it meajar / menajar/mengajar/melajar? (The correct one is "mengajar"). Is it "berajar or belajar"? (The correct one is belajar). That's really difficult. So, don't give too much focus on how the word changes from "ajar" to "mengajar", "belajar", "diajari", etc.

the easiest way to learn any languages is all the same, by practicing. try to get involved in the language you learn everyday. remember that one word each day is better than one book at once. keep practicing guys!

practise it for sure.
u can make friend then have a conversation online to make it easier