TO SPEAK UP - [verb] to speak louder; to express your opinion freely
Examples of use
Speak up please, because people at the back don't hear you.
If you disagree with the others, you should speak up now.
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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