Please say hello to your parents.

Give my best wishes to your parents.
Give my regards to your parents.
Give my best regards to your parents.

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1. Are they all the same? Can we add or delete 'best'?

2. He had lunch in a second.
(What is the meaning of 'in a second'? What English word can we use instead of that?)

Gramatically, yes on both counts, but listen to the sound of it and go with that.

"in a second" means quickly. He had lunch in a hurry.

Anonymous is correct.

Your first four sentences are all fine; you should not leave "best" out; it needs to be there.

1. The phrases "Say hello to your parents," "Give my best wishes to your parents," and "Give my regards to your parents" are all generally used to convey your kind regards or greetings to someone's parents. They are quite similar in meaning, but there are slight differences in the level of warmth or formality they convey.

Adding "best" before wishes or regards adds an extra emphasis and sincerity to your message. It implies that you are extending your warmest wishes or deepest regards.

Therefore, to summarize:

- "Say hello to your parents" is a simple and casual way of conveying your greetings.
- "Give my regards to your parents" is slightly more formal and polite.
- "Give my best wishes to your parents" is a warmer and more heartfelt version.
- "Give my best regards to your parents" combines the warmth of "best wishes" with the formality of "best regards".

2. The phrase "in a second" is an idiomatic expression that means doing something very quickly or without hesitation. It implies that the action was completed almost instantaneously.

An alternative word that can be used instead of "in a second" is "instantly" or "immediately." Both words convey the idea of acting or completing something without delay.

1. The phrases "Please say hello to your parents," "Give my best wishes to your parents," "Give my regards to your parents," and "Give my best regards to your parents" all express a similar sentiment of conveying a message or greeting to someone's parents. However, there are slight differences in the tone or level of formality.

- "Please say hello to your parents" is a simple and straightforward way to ask someone to pass on a greeting to their parents.
- "Give my best wishes to your parents" adds a sense of warmth and sincerity to the greeting, indicating that you have positive thoughts and good wishes for the parents.
- "Give my regards to your parents" is a more formal and polite way to send greetings, showing respect and acknowledgment.
- "Give my best regards to your parents" combines the warmth and sincerity of "best wishes" with the formality and respect of "regards," making it a more heartfelt and respectful expression.

Whether you include or omit the word "best" depends on the level of warmth and sincerity you want to convey in your message. Adding "best" emphasizes the depth of your good wishes or regards.

2. The phrase "in a second" indicates that someone performed or completed an action very quickly. It's used to express speed or immediacy. In this context, "He had lunch in a second" means that he finished his lunch very quickly.

Instead of using "in a second," you can use other English words or phrases that convey the same idea of quickness or speed. Some alternatives include:
- "in no time"
- "in a flash"
- "in a jiffy"
- "in the blink of an eye"
- "in a heartbeat"