Keep one's ears open:
打开耳朵 ×
留心听(看);密切注意 √
例句:
Keep your mouth shut and your ears open.
多听少讲。
Have a word in somebody's ear:
和某人说秘密话 √
例句:
When you have time would you come to my office, I'd like to have a word in your ear.
你有空请来我办公室,我想同你私下谈谈。
Couldn't believe my ears:
不敢相信自己的耳朵 ×
不敢相信 √
例句:
I couldn't believe my ears. My boss fired me today.
我不敢相信,今天老板把我解雇了。