Re: Can you please explain these English idioms/ sentences to me?
1. He could see through the door thanks to a chink in the wood.
Does it mean there was a hole in the wood, like a peephole (but an actual hole) and that he could see through it?
Yes. Or there may have been a small hole/gap between the boards which made up the door.
2. There was a clanking of couplings as the carriage was joined to the locomotive.
I don't understand this sentence at all.
A "clank" or "clunk" is the sound made when two heavy metallic objects collide. "Couplings" are the heavy metallic mechanisms which connect railway carriages and locomotives to one another. The connection is made by bumping the locomotive slowly into the carriage. Hence the collision; hence the clanking!
3. The buzz of motorcycles floated across the fields.
Were motorcycles driving on the fields or something? I don't get it. :(
We don't know where the motorcycles were driving. It may have been in the fields; more likely it was on a road beyond the fields. In any case, there were fields between the motorcycles and where the listener was positioned, the noise was heard coming from across the fields.
4. A chink of sunlight came through the curtains.
=Some sunlight filtered through the curtains?
Yes. Or maybe there were some "chinks" or small gaps in the curtains?
7. A chirping flock of small children
I don't understand this sentence at all. :( What are the children doing?
The children are behaving like a group of small birds: grouped close to one another, and talking excitedly together in high voices. Hence the metaphor.
8. So far the rival companies are level in their take-over bid. The crunch will come tomorrow at the meeting of the shareholders.
I don't get this sentence either!
The rival companies are competing, either to take control of another company, or to take over one another. A final decision - or at least a significant one - will be taken at tomorrow's meeting. In this context, "crunch" means "final resolution" or "end game".
9. The ice chinked against the side of the cocktail glass.
What is happening here? Is the ice moving inside the glass?
Yes. See "clanking" above.
10. The dog crunched the bone.
What exactly is the dog doing?
The dog chewed the bone. As the bone broke into small pieces, it made a sound known as "crunching".
11. It's easy to devalue a currency. But suppose other countries react by devaluing theirs? That's the crunch.
Why does it say "that's the crunch"?
That's yet another common meaning of "crunch", meaning "difficulty" - but you can also see that it's related to the meaning in 8. above.
12. He crunched over the hard snow (his boots broke over the brittle surface).
What is he doing exactly?
He is walking over the snow. The sound his boots make as they break through the hard surface is similar to that made by the dog chewing on the bone in 10. above.
Finally, re probus's explanation of 2-stroke engines vs 4-stroke engines: although it is correct to say that the lubrication system is different in most [but not all] cases; that is not the essential, defining difference. A 2-stroke engine completes a power cycle in one crankshaft revolution and with two strokes (up and down movements) of the piston; whereas a "four-stroke engine" uses two crankshaft revolutions and four strokes to do so.
I'm not a teacher of English, but I have spoken it for (almost) all of my life....