Past Tense

[tedtmc]

It is high time the government changed its policy on immigration to curb the massive influx of illegal immigrants.
It is high time the government changes its policy on immigration to curb the massive influx of illegal immigrants.

I understand both the past and present tenses could be used in the context of the above sentences. Why is the past tense used for something which has yet to occur?

Is there a difference between using the past and present tense?

[Casiopea]

We sometimes use past tenses to describe things in the present or future that are imagined or unreal.

It's time we went.

We are using It's time... here to say that something is not happening, but it should be happening. Compare also the following:

Read more here ...

[tedtmc]

Thanks Casiopea for the prompt response.

That is a very useful link on the use of the past tense for the present & future, something which I couldn't find in the high school grammar books.

Which is preferable though, in the context of the sentence I mentioned - the present or past tense? And what is the difference?

[Casiopea]

Here's on observation:

• "It's high time we send him a registered letter."
=> a plan to do something, a "real" condition for a future action.

• "It's high time we sent him a registered letter."
=> a regret that we haven't done something, more focus on the past non-action, and a wishful thought about the future action that may or may not take place—an "unreal" (or unsure) condition for a future action.

The issue is how fully the speaker expects the action to happen. If there's a real intention to send that registered letter now that the reminder has been issued, then the verb is present. If the speaker has some doubt that we'll ever get around to sending the letter after all, then the verb is past.

Source: The Grammar Logs. Sun, Dec 9th, 2001