Um- is used for verbs that already happened and is currently happening. It can only be used with verbs that begin with a vowel when used as prefix.
Past tense: Um + verb (root word)
Progressive: Um + first syllable of verb + verb (root word) <<< repeating the first syllable of the verb only applies when um- is used as a prefix
Umalis = left, Umaalis = leaving
Umiyak = cried, wept; Umiiyak = crying
Umakyat = climbed, Umaakyat = climbing
Umikot = turned, Umiikot = turning
*The words below involves a different set of rules but I gave this example just to show how um- affects the tense. I'll explain more about this form in another topic if you want.
Kumain = ate, Kumakain = eating
Kumilos = moved, Kumikilos = moving
Mag- is usually used for verbs in the present or future tense. May be used with verbs that begin with a vowel or a consonant. Mag- can only be used as a prefix.
Present tense: Mag + verb (root word)
Future tense: Mag + first syllable of verb + verb(root word)
Mag-alis = remove, Mag-aalis = will remove
Maghugas = wash, Maghuhugas = will wash
Mag-isip = think, Mag-iisip = will think
Magsalita = speak, Magsasalita = will speak
Note: Adding um- or mag- to the same root word completely changes the meaning of the word.
Root word: alis = go
Umalis = left (went), Mag-alis = remove
Root word: akyat = climb
Umakyat = climbed, Mag-akyat = carry (bring, transport to a higher place)
I'd be happy to elaborate further if you want to know more.