@porkandpotatoes
When an actor-focus verb has both mag and um forms, are both options equally valid for all contexts?
In general, yes, because "bum
asa" and "magbas
á" both mean "to read".
... what factors may affect the choice?
Regional/local usage and/or personal choice.
The response to your saying "nagbasa ako ng libro" in Bataan was based on their regional/local usage there.
I was born and raised in Manila as my parents were, too. "Nagbasa ako ng libro" is how we say it here. If I hear someone use "bumasa" instead of "nagbasa" in that sentence, I would think that the person is not a native of Manila or he learned Filipino from someone who hails from another Tagalog or a non-Tagalog region.
Although I can consider both words interchangeable, I would use "magbasa" to refer to reading things with words or sentences on them, which we encounter about 99% of the time.
On the other hand, I would be inclined to use "bumasa" to mean rudimentary reading, as when a child is just learning to read the letters of the alphabet or about "reading" abstract or conceptual things, e.g., the reading of numeric symbols, tarot cards, and signs/symbols like 🚸💲🚷💈♻ï¸ðŸ•˜ðŸ‡ºðŸ‡¸ðŸ›‘ðŸ‘🎵... etc.
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