If 'could' is interpreted as the past tense of 'can', meaning ability, then 1c. is incorrect. But if 'could' is interpreted as the modal 'can', potential or permission, then 1c. is correct.
1c. If she
has time, she
could go.
has (verb, present);
could (modal, present);
can (verb, present)
Note,
can means ability; informally
can means potentially:
a) She can go (verb, present, ability)
b) She can go (verb, present, potential)
c) She could go (verb, past)
d) She could go (modal, potentially)
a) and b) look the same, but they differ in meaning. c) and d) are look the same, but as part of a sentence, say, 1c., only d) is possible:
1c. If she has (present)..., she could (modal, potentially) go.
Cf. *If she has (present)..., she could (verb, ability) go.
The second one is ungrammatical (*) because the verbs do not agree in time. 'has' is present, whereas 'could' meaning, ability, is past. If we assume that 'could' is a modal, then the sentence is fine. Semantics, or meaning is how the speaker knows which 'could' is possible and which 'could' is not possible. For example:
She could. (if past, then it means, ability, and if non-past, it means, potentially)
As for the rest, please study up on modals.

You should try to use the sites that we've recommended. Once you've had a chance to review the basics, let us know and we'll start up the discussion again.