expressions – “it’s best”
Yes “it” can be used in all of those constructions and others, but soem of the examples could use adjustments.
For number 2, depending on the intended meaning I would suggest:
It would have been best to have bought it yesterday.
This implies that the speaker is talking about an alternative that did not happen, but should have — a form of the subjunctive. If the intent is to say that it did happen, and to approve of that, then it could be worded as:
It’s best that he bought it yesterday. or It’s good that he bought it yesterday.
2a has a quite different meaning, implying that what is being approved of is not that the purchase be made, but that he is the one to make it.
3 “It’s best (if) he (not) buy it tomorrow.” is not a subjunctive form, and some options do not work well.
3A It’s best he buy it tomorrow.
the verb tense is wrong with 3A. Better would be:
3A1. It’s best that he buys it tomorrow.
This form assumes or suggests that the purchase will happen, and approves of it.
3A2 It would be best if he buys it tomorrow.
This makes no such assumption. Indeed it suggests doubt over the event.
3B “It’s best if he buy it tomorrow.
again does not work. In general the form “he buy it tomorrow” has the wrong tense. The apparent sense of 3B could be expressed with
-
3B1 It would be best if he buys it tomorrow.
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3B2 It will be best if he buys it tomorrow.
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3B3 It’s best that he will he buy it tomorrow.
3B1 expresses doubt over the event, indeed expects it not to happen. 3B2 expresses uncertainty, it might or might not happen. 3B3 expects that it will happen.
The variants of 3 including “not” simply change the event from buying to not buying, but are otherwise the same, and the same options are available.
4 It’s best if he bought it tomorrow.
again has the wrong tense. This should be one of the 3B variants (3B1, 3B2, or 3B3).
5 It’s best if he had bought it yesterday.
This should not use “It’s” which is after all simply “it is”. Instead this idea CAN BE expressed by the adjusted form of 2 above.
6 It’s best if he were buying it right now.
should use “would be” instead of “is” (here ‘s), so:
6A It would be best if he were buying it right now.
6A is a subjunctive, because it implies that he is not doing so.
As for 7, use 3A1 instead.
