The differences

Over on the ETJ list, we had the following question:

A JTE asked me if I could explain the differences/similarities between:
(be over & end)
(call for & need)
(start & leave) as in:The best thing is to (start / leave) at once.

Rather odd question, I thought, but I'll give it a shot.

As for call for vs. need, perhaps the difference can best be explained by contrasting them in
"The forecast ________ more rain." The similarity is that they are both completely ungrammatical in that context, so you need the third person 's', producing

1st person: "The forecast need more rain."
2nd person: "Hang on! That's not grammatically correct."
3rd person: "Shhhhh! That's not PC anymore."

Moving on to be over and end, though very similar, one can detect a slight change in nuance, which is brought out nicely by putting them in the following context:

"the dog bit me on my rear _____"

In the final pair, there isn't enough context to really tell start and leave apart, but if we add one sentence to the beginning, it becomes much more obvious, to wit,

"I think I'm going to fart."
"The best thing is to start / leave at once."