It's a nice question and quite hard to explain in English. :)
But, let me try it. (Others are welcome to correct me if i'm wrong)
"lupa" is a verb.
Indonesian language has "imbuhan", that contains :
1. awalan : you have to put "mem/ber/di" etc + word
example : mem+baca = membaca
2. sisipan : you have to put "el/er" etc between the word
example : gigi+el = g(el)igi
3. akhiran : you have to put word + "an/kan" etc
example : lupa+kan = lupakan
when you put "imbuhan" on word, you've change its meaning.
lupa : verb
- Ooo, saya lupa! / Ooo, i forgot!
lupakan : verb + kan
- Lupakan hal itu / Forget it! --> It became an imperative sentence.
melupakan : me+ verb + kan
- Saya melupakan kata sandi saya / I forgot my password.
I hope that helps.
*I really want to know the whole sentence from each word and where did you hear that. Words can have different meaning in different context, right?
And I guess, when you learn Indonesian language, your question is on advanced level. Even for us, the native speakers.
Good job and have a nice day. :)