Listen carefully, now.
Meaning: This means to pay close attention to what someone is saying at the present moment. It's like saying, Give your full attention to the person who is speaking.
Choice of Words: Listen is a verb that means to pay attention to sound or someone speaking. Carefully is an adverb that means to do something with caution or attention to detail. Now is an adverb that means at the present time or immediately.
Alternative Expressions
Related Expressions