I'm not a native tagalog speaker, but I know enough to answer your question. Pag is a prefix that basically turns a verb into a noun. For example, Pagmamahal is translated as "loving or love". It is not used as an adjective though, i.e. it does not mean "her loving arms". It is used as a noun. For example, "her love of flowers". Actually, for love, it is better to use ibig, as in pag-ibig. Mahal can also mean expensive so some confusion can occur if used as pagmamahal. Magpa is a prefix that indicates the habit of doing something. But be flexible with the interpretation. Mapagmahal means the habit of loving, which really means a loving disposition. Mapagbigay means the habit of giving, which is translated as generous.