Is there any difference between:
- If I were you, I would work harder.
- If I were you, I would have worked harder.
And can we use was rather than were in both sentences?
- Is there any difference
If I were you, I would work harder is subjunctive present, which means that the verb phrase “would work” expresses something that is a possibility, a wish or a desire rather than a fact (a fact is expressed using the indicative mood). The need to express this as a possibility rather than a fact is where we get “would” from, and “would” is not your problem. “Work” is present tense, so saying I would work harder means that if a certain condition is fulfilled, then what will occur is me working harder right now. The action occurs now, but only if something else happens too.
If I were you, I would have worked harder is subjunctive present perfect, which means that the time frame for this action indicated by the tense is before now, but could be continuing in the present, or is unfinished. (I have lived here and I have read a book and I have just finished my homework). Essentially, if the condition is fulfilled, then the action has already happened.
The difference is that one implies that if you were he, the action would occur now, whereas the other suggests that if you were he, it would already have happened.
- Can you use ‘was’ in both sentences
Traditionally, you cannot use ‘was’ in either sentence because of the subjunctive mood. In English, the subjunctive is nearly extinct, but its verb form for to be (past) is the same for all persons and numbers:
If I were / If we were
If you were / If you were
If he were / If they were
However, some authorities suggest that this is formal, written or archaic, and that the use of ‘was’ for first and third person singular is acceptable. I tend to lean towards tradition, but I suppose the choice is yours.