Do you think it's rude to constantly correct someone's English?

Yes and no!! Unsolicited correction could lead to unpleasant situations if it is done in a patronising "I am smarter than you" manner. I have worked with a number of different nationalities and my approach to the way they speak English is to not interrupt them as they are speaking if, what they are saying makes sense although not necessarily grammatically correct! In a quiet moment afterwards I will point out where I think they have been going wrong. IF they want me to! I have yet to meet anyone who has resented this approach or turned down the offer of help, suggesting in fact that they want to learn and learn the correct way. We all need help when learning something new but help should be given in a constructive and sympathetic manner.

Yes it is. It is better to lead by example.

well your question is very good, and to answer it I'm going to say it depends on the way you are correcting them, your tone, the person you are talking to (friend, professional, or a student), and the situation. 

If you are talking to a student or someone English isn't their first language you could go two ways, use passive correction, which means you WAIT until they are done talking and nicely point the mistake in a way that makes them feel it is constructive criticism.

OR, you could do the immediate correction (((and you have to be very careful with this one))) DON'T interrupt anybody while talking just to correct them, especially if it's something small and you could let it go for another time.. Some people fix their own mistakes the more they practice, and some people make mistakes when they are nervous. So, please keep that in mind. 

If you are in a professional setting, Don't embarrass them in front of other just keep the conversation going and enjoy and make not of mistakes later in a NICE way.. again make them feel this is Constructive criticism and nothing to hurt or humiliate them. 

Even with friends you have to be careful.. I witnessed this happen with my friends where one was constantly interrupting the other and get off subject just to correct small mistakes until my first friend (with the mistakes) stopped talking as much feeling that the other is going to keep a close eye on what he/she saying. that was very a very rude way to do it. So, again be careful with the way you are using. 

Best of luck, 

Wafa R. 

It depends on situation on which stage you should take this step.

Usually in daily conversations in a work or other professional contexts, people don't correct each other as long as the communication is going on easily. Even when the message is not clear because of a grammatical or lexical mistake, the receiver would usually ask for clarification rather than trying to correct the speaker's English.

Even in a completely different context, like learning in a classroom, teachers don't tend to correct every single mistake so they don't discourage their students from participation, and more importantly, learners need time to improve. A lot of people made mistakes, not because they don't know what is correct, but because it takes time to improve your language skills. 

So I would say let it go as long as you get what the speaker is trying to say.

if u r taking about a student then in that case it is difficult so immediatly but in case of profficinals1 .means want b1 of them then it may have great chances 

because by this we will also be able to improve our english speaking

Yes. But you can use some polite expressions before correction like ; you mean that, you want to say that but afer he finishes his speach .So that you can help them learn E at the same time you won't make him get embarrassed 

Yes, if its done in a front of other people. It should be done in private, face to face. Then you'll certainly achieve two things;1. You ensure that you won't put  them in a uncomfortable situation and2. You assure them that you have respect. And this should be done in the best manner.

This is a principle for every situation when it comes to correct somebody's mistakes, not only in a language matter, because after all every single one of us makes mistakes. Of course, all of this depends from the relations and the interactivity that you have (or you have built) with those people.

The respect, honor, attention and care of other people towards you is in a fact and depends always from the respect, honor, attention and care that you show towards them.

Peace and respect...

Yes it is.

If it needs to be rectified then the correct sentence to be said in the next conversation so that the message is received without putting the person in an embarrassing position.

Correcting soneone's English is fine but constantly correcting it makes the other person doubt his own capability to learn and understand English. Do not force or pressurize the other person while he is in the process opf learning English. Give them a free space, let them learn on their own and try to be friendly while correcting their English.