1. You're right about this. You can say:

It's time the children went to bed.

It's about time the children went to bed.

It's high time the children went to bed.

You can't use the 'should' form here. Only the past is possible. But note that the 'that' is optional.

2. You're right that 'If only ..!' means 'I wish it were so!', but that isn't what 'I'd rather' means . 'I'd rather' and 'If only' are used to express different ideas.

a) 'If only you wrote to me!' means that you don't write to me, but I wish that you would write.

b) 'I'd rather you wrote to me' is a polite way of saying that I don't want you to speak to me in person or to phone me. It is probably referring to the future. For example:

'Shall I come to your office and tell you my proposal?'

'No, I'd rather you wrote to me. I'm very busy at the moment.'

Only the 'If only..' sentence can be paraphrased as 'I wish that you wrote to me'. Both 'If only ' and 'I wish' express a dissatisfaction with a real and present situation and a desire for that situation to be different. 

'I'd rather' isn't used in this way, and your 'I'd rather' sentence can't be paraphrased as 'I wish'. It could be 'I'd prefer.. 'I want..' or 'I'd like..', but not 'I wish'.

hi su.ki. thanks for your reply. it's all clear now.

No, that doesn't work with a gerund, I'm afraid. If you want another alternative, though, you could use an impersonal 'it' and a second conditional:

'I'd prefer it if you wrote to me.'

hi, su.ki. thanks for ur explanations!

gosh, i've mixed up the english grammar with the french one then! what a shame! 

as to your example - <em>i'd prefer you to write to me</em>  - would it be the same to put it like "<em> I prefer your writing to me</em>" ? 

1. If only you wrote to me. = I wish that you would write to me. = I wish that you wrote to me.

--- are they all the same?

Yes, but the second one gives more information. It implies that you are unwilling, reluctant or just too lazy to write to me. We often use the 'I wish you would..' form when we are trying to persuade someone to do something. The implication here is that it is something within your control.

2. I'd rather that you wrote to me. (instead of coming directly to me and telling me). = I'd like/prefer that you wrote to me. = I want that you wrote to me.

--- are they all the same?

A few inaccuracies here. The want/would like sentences need infinitive constructions, not subjunctives i.e.  'I want you to write to me' and 'I'd like you to write to me'.

NB They'd be subjunctives in other European languages ( 'Je veux que vous écriviez', if I'm not mistaken).  This doesn't work in English, though. It has to be [want] + [object] + to + infinitive.

With 'prefer', you can use either construction. Either 'I'd prefer you to write to me' or 'I'd prefer that you wrote to me'.