Magandáng umaga: good morning.
Good/beautiful on its own would be magandá.
The root is gandá, beauty.
Some learning web sites could write it as "Magandng umga".
If these two underlines are for stress (emphasis), then I wonder what the difference between "a" and "á" is.
Or is the underline used for long vowels instead?
Returning a greeting can be done by repeating it and adding "sa iyó".
"sa" means "to" (as in "to you"). It also is used for several other meanings. Because prepositions don't always translate too well.