SINCERE - [adjective] true, real or honest
Examples of use
I'm sure he was not sincere in what he said to me last night. He often tells lies.
He's a great actor. He's so sincere in what he does!
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English speaking practise
Remember that if you don't use that word in English conversations or writing you'll forget it in a short time. So the best thing you can do to learn this word is just trying to use it in speaking or writing. Remember also that practise is the key to fluent english speaking.
You can always find a partner, who would also like to improve his/her