If I could offer one piece of advice to non-native speakers in a meeting with mostly native English speakers, I would say, “Open your mouth wide — yes, wide — when you speak.”
In many cultures, such open-mouthiness is considered unnatural and, frankly, vulgar. The English TH sound is many foreign learners’ worst offender, making speakers stick their tongue between their teeth. But most other common English sounds, found in words as basic as “and,” “off,” “I” and “out” also force the mouth to open. A strong, positive, “Yes,” forces the mouth to open, as do the two negatives, “No,” and “Not.” (“Not” rhymes with “watt,” not “cut”. )
English speakers think nothing of showing teeth and tongue when talking: It’s expected.
Foreign speakers, on the other hand, often bring their native language habits to English and keep their mouths relatively closed. But in a meeting with mostly American speakers, a semi-closed mouth could get in the way of being understood.
Say someone asks you (a non-native speaker) a question. You reply with a simple “yes” or “no,” with either word spoken with a tense mouth and barely open lips. A usual response from colleagues at the table might be, “What?” or “Should I repeat the question?”
The confusion among American colleagues comes from their expecting to hear, or at least lip-read, a firm YES or NO — with both words requiring the mouth to move and open. So a foreign speaker who replies with a mouth that barely moves probably won’t be understood — either not heard at all, or thought to have not understood the question.
To help you start opening your mouth when speaking English, try watching and imitating Americans — in public, private, or on video: watch how their mouths work and then practice: for example, when you hear someone say, “Great!” or “Okay?” or “Fine!” repeat the word — and open your mouth wide. Try saying other basic words and work up to such common open-mouth phrases as, “HAMburger — rare, please” or, “How ARE you?”
While practicing, exaggerate your words and look in a mirror. It’s hard to overdo it in English. After a while, using the muscles in your mouth will help you be better understood — and the looks of happy comprehension from your colleagues should keep your open-mouth effort going.