Hi Martha,

One way to look at this is to remember that subjunctive mood is not a tense.

Look at sentence (1).

1. Could you climb a tree yesterday?

This has a couple of possible meanings, as shown in a and b.

a. The person attempted to climb a tree and the interrogator wants to know
if the attempt was successful.
 
b. The interrogator wants to know if the person possessed the ability to
climb a tree(whether an attempt was made or not.)

In sentence (2) we have:

2. Could you climb a tree today?

This also has two possible meanings as in c and d. 

c. The person attempted to climb a tree earlier on the same day, and the
interrogator wants to know if that attempt was successful.

d. The interrogator wants to know if the person has the ability to climb a
tree the same day. 

We see similar differences of "could" in e and f.

e. Could you swim when you were young?

f. Excuse me, could you help me find something?

Hope this is of some use. 

All the best,


Til Turner
Northern Virginia Community College

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