In your example, Somanath, there is very little difference in usage and both are, of course, correct. You might argue that ask is more polite as asking for something is the same as requesting it, whereas telling someone to do something is the same as instructing or ordering them, but in this example either is appropriate. In the following example too, you can use either tell or ask:

• Why don't you come round at about eight for a bite to eat?
  ~ I told them to come round at eight for something to eat.
  ~ I asked them to come round at eight for something to eat

But sometimes we need to use tell when the meaning is more explicitly instruct and ask when the meaning is more explicitly request. Compare the following:

• Be careful not to dive too close to the rocks.
  ~ I told them not to dive too close to the rocks.
  
  
• Don't drink water from the stream. It's polluted.
  ~ I told her not to drink water from the stream. It's not clean.
  
  
• Can you show me how to operate this computer?
  ~ I asked her to show me how to operate the office computer.
  
  
• Would you let me know when Tony arrives?
  ~ I've asked him to let me know when Tony arrives.

Verb + object + to + infinitive

In English, there are a wide variety of verbs that have to do with requests, advice and instructions that follow the verb + object + to + infinitive pattern:

Note the difference in meaning in these examples of use:

• I always advise my students to have a good night's sleep before an exam.
  
  
• I begged her to let me see John before he left, but she refused.
  
  
• He's got a keyboard exam coming up soon, so I try to encourage him to practice for half an hour every day.
  
  
• She's not in the office at the moment, but I'll get her to phone you as soon as she gets in.
  
  
• When I instructed them to stop writing and put their pencils down, they just carried on as if they hadn't heard me.
  
  
• I'm going to invite my Chinese friend to spend Christmas with us.
  
  
• The gunman ordered the women to get out of the car.
  
  
• I can't persuade my son to have regular dental check-ups and dental care is so important.
  
  
• She reminded me to cancel the newspapers before I left on holiday.
  
  
• I have warned them not to swim in the sea when the red flag is flying, but they take no notice.

Note that we also use tell with a that-clause as well as with the to-infinitive pattern. Note the difference usage in this example.

• I've told my wife that I shan't be home for dinner this evening.
  
  
• I've told my wife not to expect me home for dinner this evening.
  

Note that we also use ask with an if- or a wh-clause when we are reporting yes/no-questions and wh-questions:

• Can I go home now? I've finished all the work you've given me.
  ~ I asked my boss if I could go home as I had finished all the work he had given me. But he said, no, I couldn't!
  
  
• What sort of food would you like me to prepare for the party on Saturday?
  ~ I asked them what sort of food they wanted for the party on Saturday
  
  
• When would you like to have it delivered?
  ~ I asked them when they wanted to have it delivered.
  Let me know by lunchtime on Friday.
  ~ I asked them to let me know by midday on Friday.
  
  
• Where do you keep your jewellery?
  ~ I asked where she kept her jewellery, but she wouldn't tell me where.

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