Learn English – I’d rather + past perfect/present perfect

I use just very often in speech and casual writing. I tend to edit it out of more formal writing. I am a native American English speaker from Tampa and Boston.

To me, it can have both meanings, and I don't think that I have a preference. Your examples are all ambiguous to me. I think that the meaning is often clear from context (don't underestimate the power of context!), as in:

1) I'll be there soon. I just woke up. (recently)

2) I didn't call her. I just sent her an email. (merely)

3) I tried, but I just don't understand. (simply)

I don't see how changing tenses or aspects can help you, except that the future and simple present (always?) rule out the recently interpretation. But all of the following are equally ambiguous to me (sans disambiguating context).

4) I [just ate/am just eating/was just eating/have just eaten/etc.] an apple.

When I want to be crystal clear, I just replace just with merely, only, simply, or suchlike. I think that I less often change just to recently. I think I more often say just now to indicate that I mean recently.

In your examples, 1.a is correct because it is referring to what the subject, or person, is current hearing. it is still in progress. 1.b would mean the same thing as " I can't believe what I am able to hear from here". It does not refer to what you have heard, instead it refers to what is possible to hear.

2.b is more natural and you are correct in that scenario. What you should consider is that since it is a movie, they may have only shown the character witness the dark entity once but still could have seen it after as well.

In example 3, both are correct but differ in meaning. 3.a means that the subject is still seeing it. Also, it could be referring to something specific. With 3.b, it is more open ended so the question could be general, as in nothing specific.

example 4 seemed to have been well answered in the other forums. I hope this was a little helpful.