What's your advice to an English teacher who wants to teach (English for Specific Purposes) when they have little knowledge of THE specific purpose?

English for Specific Purpose usually refers to design English course for specific field, for example English curriculum for vocational high school of engineering arranges the English materials which relates to engineering terminologies. So my advise is to understand well about the specific field itself. Thank you.

To know specific purposes, you have to learn first and see what exact specific purposes you're gonna teach in your nearest future. So, first chose the right faculty, learn there and then voila, you know what to teach and who to

I feel Grammars are the base of English, so be it any specific purpose or the most crucial English, it all comes down on grammatical part and vocabulary.

He is better get well informed about the purpose (say Business Englsih for example) and nature of language used, before proceeding to teaching; otherwise a question - to teach what - will arise!

1. Know the terminology used in that particular field.

2. Know how and when it is used.

3. Research when your tentative learners will use those terms and in what situations.

4. Conduct a survey among the would-be learners to know what they really wish to learn.

5. Find out from the concerned company what they want their employees to be proficient at.

Better don't teach if you don't even know what you are supposed to teach. The specializations, like, ESP, EAP, Business English, etc. are created for ease of expertise, so, a teacher with the basic qualification to teach English will find it difficult to cope with the terms and concepts used in the specific subjects and registers, like, the register of Law, or of Medicine, or of Engineering, unless he/she is trained in the specific field too. 

Confidence in your own abilities are encouragement and you can say, "To what heights I can not rise". 

Learning about specific purpose before starting to teach is one of the options, provided you are enough confident about your abilities.

If you can't learn this specific purpose or you have learned but you are still in doubt about its perfection then it's better to drop the idea of teaching English for a specific purpose.

Research the different types of English for a specific purpose and decide which one interests you. For example Business English, English as a second language,or written English.  TESOL  offers certification classes for different types of specialized English. Bridge TEFL is another institution that offers English classes that may help you decide.

He or she must get to know the purpose first or else they won't be able to deliver the rules of the language as it should be delivered. also, they will lack the ability to actually teach what they should be teaching.