My English teacher was explaining simple past and present perefect tenses and he threw the following sentence
I'd better ring him
I thought he meant "I would", but he said it is I had. I don't understand how it makes sense, had must go with the third form of the verb rang. It is similar to saying
I had better left the door open yesterday
I had better eaten the food in the morning -- I wouldn't be hungary now.
It sounds like a subjunctive mood situation. My teacher told me, it makes sense to him and gramatically it is a simple past tense not a present perfect tense. He is a native kiwi/Aussie English speaker.