when you doubt something you believe it to be false, as when you suspect something you could be thinking it is true or not. For example in sentence 1 you could say I suspect he can't do the job, meaning the same as doubt he can. In sentence 2 if you were to say, I doubt they're a couple, you mean to say you think they are not. In sentence 2 if you said 'suspected they are not a couple', it would be like saying 'he doubts they are a couple' I hope that helps, If that doesn't make sense let me know and I'll try to explain it again.

I doubt - I'm not sure, It can't be true. I suspect - I think, It can be.

"Doubt" means you think something is not true. "Suspect" means you think something may or may not be true, especially if someone is telling you the opposite. 1) I doubt he can run fast. That means I don't think he can run fast. 2) I suspect he can run fast. That means I think he can run fast, even if he has told me he can't. 3) I suspect he can't run fast. This means you think he can't run fast, even if he has told you he can.

suspect and doubt are quite different. If someone "doubts" that a certain event will occur then that event is unlikely to occur (according to the person who doubted it). For example, "i doubt the world will end tomorrow", means I think the chances of the world ending tomorrow are unlikely. Suspecting something is almost like the opposite. When someone is suspecting they are making an inference. "i suspect the world will not end tomorrow", this sentence is similar to the one using doubt (notice the not). in your example 2 Richard thinks the two people are a couple, if the word "suspected" were replaced with "doubted" then Richard would think the two people were not a couple (the sentence would not make much sense then because the beginning almost infers they are a couple because of their close proximity).