[译] 那部机器的故障原因还未确知。
[误] The cause of trouble with the machine is not known for a certainty.

[正] The cause of trouble with the machine is not known for certain.
[注] for certain ( = surely, without doubt) 作"确实地"解，可用于肯定句或否定句中，如: I cannot say for certain when he will arrive. 我说不准他什么时候会到。/He will be there for certain. 他肯定会到这里来。for a certainty ( = without doubt) 意为"确凿无疑地"，只用于肯定句中，如: I know for a certainty that it is hard to study English. 我确实了解学英语不容易。