Синонимов много можно подобрать к данным словам и выражениям, все зависит от ситуации. По три синонима или синонимичных выражений приведу к каждому:
1. to sleep – to go to bed, to sack out, to go to rest
2. to have serious problems – to be up against, to be faced with a difficulty, to be in the soup
3. garbage – rubbish, litter, trash
4. to forecast – to predict, to foretell, to prophesy
5. to understand – to comprehend, to realize, to catch on
6. speed – rapidity, quickness, swiftness
7. people – men, folk, human beings
8. condition – factor, situation, status
9. perspective – prospect, outlook, future plans
10. ecological – environmental, bionomic, eco
11. to throw away – to put away, to discard, chuck away