Look at the Grammar box. Then compare the first sentences in 1-5, which contain a modal verb, with the changes in the second sentences. What are the differences in meaning?
1 You might stop weak species going extinct, which could be a bad thing.
You will stop weak species going extinct, which is a bad thing.
2 Maybe we shouldn't interfere.
We mustn't interfere.
3 The survival of the fittest' can suggest evolution is a kind of competition.
'The survival of the fittest' suggests evolution is a kind of competition.
4 If that habitat disappeared for whatever reason, they'd easily die out.
When the habitat disappears, the animals die out.
5 Shall we leave it there?
Could we leave it there?
Modal verbs and meaning
A modal verb (would, will, may, might, could, can, should, shall, must) adds a general meaning to another verb to show a speaker's attitude or intention.
The first thing that will strike people is ...
= I am certain it strikes people.
The first thing that should strike people is ...
= I believe it strikes people, but I'm not certain. Modal verbs can also be used to express: certainty, uncertainty, obligation, permission, suggestion, possibility and frequency (habit).
Hugh Dellar, Andrew Walkley