Learning From The Movie UP

Have you ever seen the movie UP? It is one of my favorite movies of all time! The characters are cute and the movie carries a lot of inspirational themes. One important lesson from the movie is that life is full of adventures, don't wait till it's too late. Sometimes life can bring you unexpected sorrow but it can also bring you unexpected people and opportunities. So be ready for adventure everyday!

There is one particular scene in the beginning where Carl and Russell meet for the first time. This is what they say to each other:

Russell: Good afternoon. My name is Russell and I am a Wilderness Explorer in Tribe 54, Sweatlodge 12. Are you in need of any assistance today, sir?
Carl Fredricksen: No.
Russell: I COULD help you cross the street.
Carl Fredricksen: No.
Russell: I COULD help you cross your yard.
Carl Fredricksen: No.
Russell: I COULD help you cross your porch.
Carl Fredricksen: No!

Do you know why the young boy Russell uses "could" instead of "can?"

CAN
We use "can" to talk about the present tense for ability (being able to do something or knowing how to do something)
"I can read", "She can speak French", "Can you swim?"

COULD
"Could" has several functions:

It can act as the past tense of "can"
"Last year you could get a table with no reservation!

It can also be used to talk about permission in the past or future.
"Could I borrow your car tomorrow?"

Russell in the movie uses "could" to express present possibility or options.
"I could help you cross the street"
"We could go for a walk after dinner"
It is not definite but it is a suggestion. There is the option and possibility  for now or the future.

You could also look at a similar past lesson on modal verbs here