We can use modal verbs to describe how certain we are about something.
50% / 60% - could - The door could be broken
30/40% - may - The keys may be lost
10/20% - might - He might be a mechanic
0% - can't - He can't be the owner
What do you think is happening in this situation?
You can also use modal verbs to speculate about the past. In this case use the modal verb plus have plus past participle
See here for more examples
and practise here
For more grammar exercises on modal verbs, see this book
Before an important meeting or conversation, we can imagine what type of words or phrases we will hear, for example if you have an interview you can think about the questions the interviewer can ask you and you can prepare some answers. You can use the same skill for English.
Here you can read some information about how PREDICTION can help you to understand better.
https://drive.google.com/file/d/0B1_B2HZ_bIslV0lLLW1nQnVQTWc/view?usp=sharing
You can try out this skill on listenaminue.com.
Before trying the listening gap fill exercise you can guess which words may be missing from the text based on your knowledge of the situation or of grammar and vocabulary.
Then compare your answers with the tape script. You will understand that it is not always neccesary to understand all the words to understand the general sense.
You can practise predicting and then listening on Page 41 of PET Result, exercise 1 and 2.