Question

1. Mom will be downstairs.

2. Mom will be downstairs now.
3. Mom will be downstairs in the afternoon.
------------------------------
In #2, Mom is downstairs, I think.
In #3, it tells the future tense.
Then, what about #1? Does #1 mean #2 or #3? Does #1 tells the guessing of the present or the future?

Answer 1

Yes, 1 and 2 mean, essentially, the same thing. We might say that it is likely that Mom will be downstairs IF it is the time of day (or night, whatever) when she is usually downstairs. Assume Mom stays in her bedroom upstairs until 10;00 am. If it is after 10:00 am, it is likely that she will be downstairs, but the speaker is away at school or work, so cannot verify. All three are guessing at the present and/or the future. If Mom follows her usual pattern, she will be downstairs at that time today, tomorrow, and most days.