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Hi. I think both versions, the sentences with the modal verbs "can" and "could", mean pretty much the same in meaning (not sure, though). What do you think? I also think we can replace the underlined parts with the phrase "will be able to." Also, does the phrase "be able to" denote ability? If we could replace the underlined parts with the phrase "will be able to," could we take it to mean that the "can" and "could" uses in sentences below denote abilities too?

1. Help us so we can/could get out of this financial rut we are in.

2. Send us your replies so we can/could make necessary arrangements for the birthday party.

3. If you help us, I think we can/could make it out.
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Use only can after an imperative and so (that) -- not could.
Your first two sentences are imperatives with so.

Leave me alone a while so I can think. (not could)
Please get a pen so Katy can write a check. (not could)
Stand aside so the police can get through. (not could)

You can substitute will be able to for can in any of these, but it sounds somewhat unnatural because there are so many more words than necessary.

CJ
Comments  
"could" doesn't work in #1 or #2. You may hear it used in sentences with the pattern of #3, but better is:

"If you help us, I think we can make it out."

"If you helped us, I think we could make it out."

"will be able to" is possible in all sentences, with much the same meaning as "can". To me, it seems a bit klunky in #1, however.
 CalifJim's reply was promoted to an answer.