"Mag-" is a verb prefix. We have root words, i.e. bigay, punta, aral which by themselves are not verbs and we then add "mag-" to transform them into verbs. "Nag-" is the prefix for incompleted ("present tense") or completed ("past tense") aspects of the verbs. To give you an example: ROOT: ARAL mag-aral = to study nag-aral = studied nag-aaral = study, studies, is studying mag-aaral = will study In your sentence, there is no need to use "ng" before "mag-aral" because "ng" is used before nouns only. Hope this helps.

In your example you can use it like "nag-aaral ako magtagalog" you can remove "ng" try to read my example it's good to hear right?. Anyway let's move on. Nag is can also use as present tense if you will add another 1st syllable/letter of the word= example (nag-aaral/studying, nagluluto/cooking) Nag can use past tense= example (nag-aral/studied, nagluto/cooked) Mag can use for present tense in most of it we using it as a commanding= (kailangan mong mag-aral/you need to study) Mag can also use as future tense (mag-lalaba na ako bukas/I will do laundry tomorrow) Simple tip: Nag-aaral vs. nag-aral just remember when you repeating it always remember you haven't done that but if it is "nag-aral" only it means that you already done that same as when using mag.