could - was able to - managed to

My question is: When should I use could, was able to and managed to? Please help me.

could

Could can be used in many different ways, to ask for permission, to make a request or to express ability when referring to the past. Was able to is sometimes used as an alternative to could when we are discussing ability or possibility. We tend to use could when we are talking about ability generally. Compare the following:

• By the time she was seven, she could already speak three languages.
• She started the viola at the age of eight and after only six months she could play it quite well.
• Her brother Jack was an excellent swimmer. He could beat anybody in his class.

was able to / managed to

We tend to use was able to or managed to if we are talking about what happened in a particular situation or are referring to a specific achievement:

• Were you able to / Did you manage to speak to him before he left home?
  ~ No, I'm sorry, I wasn't able to / didn't manage to reach him.
• The fog came down and I wasn't able to / didn't manage to get to the top of the mountain.
• My brother wanted to carry on, but we managed to / were able to talk him out of it.

However, with verbs that refer to the five senses, see, hear, smell, feel, taste, and with verbs that refer to thought processes, understand, believe, remember, decide, we normally use could, even when we are talking about specific occasions:

• He was standing very close to me and I could smell the garlic on his breath.
• He asked me when Julie's birthday was, but I couldn't remember.
• I couldn't decide whether to ask him for a lift or not.
• I could see that he'd been running.

unable to / not succeed in

Note that unable to is an alternative negative form of not able to and succeed in is a slightly more formal alternative to manage to. But remember that succeed in is followed by verb + ing, rather than verb + infinitive:

• We were unable to leave the room until the locksmith arrived and succeeded in unlocking the door.
• I was unable to complete the report as several pieces of information were missing.
• Having obtained them, I succeeded in completing it after a further two days.

be able to

Note that can has no infinitive form, no -ing form, no perfect form and no future form. It cannot follow another modal auxiliary verb. On all of these occasions, we have to use be able to instead. Compare the following:

• I'd like to be able to swim like Jack. He swims like a fish.
• When I'm at the sea-side, I enjoy being able to take a swim every morning.
• Unfortunately, Jack hasn't been able to swim since his accident.
  Lets' hope he'll be able to resume his daily swimming training soon.
• I'm not a member, but can I swim in this pool? ~ Why don't you speak to the secretary? She may / might / should be able to help you.

manage = succeed / cope

We use the verbs manage to and manage a great deal in current English when we want to say that we are able to cope with a difficult situation or find time for a particular task. Compare the following:

• It was very icy, but I managed to keep the car on the road.
• Veronica was very upset when Ben left her, but she managed to smile nevertheless.
• She didn't really want to go to Mexico, but Tony managed to persuade her somehow.
• Can you help me put up my new shed? ~ I can manage a few hours in the morning, but I'm busy in the afternoon.
• This is an ideal job for those who can only manage a few hours each week.
• Can I give you a hand with that? ~ No, it's all right. I'll manage./ I can manage.