Hypothetical situations

Wish and if only
Wish or if only can express how we would like things to be different if we had the power to change them.

Present states
We use wish/if only + past simple to express wishes about present states. Stative verbs such as have, be, know and understand are used.
I wish I was/were smaller.
If only I knew how to play the guitar.

Present actions
We use wish/if only + would when we want something to happen or someone to do something. Active verbs (verbs describing action) are used.
I wish you would turn your music down.
If only this wind would stop blowing.
Wish/if only + would is used if we want to express irritation at other peoples' actions or behaviour.
I wish you would stop speaking.
Wish/if only + past simple can also be used if the actions occurs habitually.
I wish you didn't (or wouldn't) drink so much.

Present ability
Wish/if only + would is used for events which are outside of our control. Consequently, we do not normally say "I wish I would". Instead, we use "I wish I could" to indicate our inability  to change things.
I wish I could remember where I put my keys.

Wishes for the future
Wish/if only + would or could can be used to express wishes for the future. This use of wish suggests that the action will probably not happen.
I wish I could go on holiday with you in winter. (I know that I can't go with you)
If there is more possibility that the action will happen, we use hope.
I hope I can go on holiday with you in winter. (I don't know if I can or not)

Past situations
We use wish/if only + past perfect to express wishes and regrets about the past.
I wish I hadn't left school when I was 15.
The following structures can also be used to express wishes and regrets about the past: Third and mixed conditionals (If I hadn't been so tired, this wouldn't have happened.); Should have + past participle (We should have got the train. This traffic is awful.)

Would rather
We use would rather + past simple when we want someone else to do something in the present or future.
I would rather you start to study now.
If the subject of would rather and the following verb is the same, we usually use the infintive without to.
We would rather sit in the garden than go to the city centre.

It's time 
We use it's (high/about) time + past simple when we want something to happen or be done now. It implies that the action should have been done already.
It's time you went to the Hospital. 

Expressing ability

Can and be able to
Can and be able to are both used to express ability. However, can only has present tense (can) and past tense (could) forms. If another form of the verb is required, be able to is used.
Present: She can/is able to speak two languages.
Past: She could/was able to read when she was three.
Infinitive: I'd like to be able to run.
Present perfect: She's never been able to save money.
Will future: He'll be able to drive when he passes his test.

Present ability
We use can or be able to to talk about present abiity. Be able to is more formal than can.
The negative form of can is can't or cannot. To form the negative of be able to, not is used before able. You can also use be unable to.
Many of my relatives aren't able/are unable to pronounce the word "taught" correctly.
Be capable of + gerund can also be used to express ability. It means to have the ability, capacity or potencial to do something.
The team has some excellent players and is capable of winning the championship.
The negative form be incapable of + gerund can be used or not can be placed before capable.
He is incapable/isn't capable of looking after himself.

Past ability
When we talk about general ability in the past, both could and was/were able to are possible.
He could paint really well when he was a child.
Bill wasn't capable of making toast without burning it.
When we talk about ability to do something on one occasion in the past, could is not possible. Instead, was/were able to, managed to + infinitive or succeeded in + gerund have to be used.
We managed to/were able to speak to Bill last night.
However, could can be used for ability on one occasion when it is used with verbs of the senses: see, smell, feel, hear, taste, sense.
I knew my husband has arived; I could smell his perfume.
When we talk about inability to do something on one occasion in the past, couldn't, weren't/wasn't able to, didn't manage to and didn't succeed in are all possible.
I couldn't do the homework last night; it was too difficult.

Verbs followed by preposition + gerund

These verbs can be followed by for + gerund:
apologize, blame, arrest, praise, forgive, punish, tell off, thank
Have you apologized to Sam for breaking her pen?

These verbs can be followed by from + gerund:
stop, prevent, discourage
His parents prevented him from leaving the town.

Congratulate and insist are followed by on + gerund.
She insisted on paying for the drink.

Warn can be followed by against + gerund.
We tried to warn her against driving in the snow.
Warn can also be used with of or about + noun.
Posters have been put up warning people of the dangers of smoking.