- If you do a favor for someone, forget it. --- إذا اتخذتم عند رجل يداً فانسوها

 If you do a favor for someone, forget it. This was said by one of the wise men of Arabia to his sons. Abu Ubaid said: He meant so that you would not feel indebted to people in your hearts, nor mention it with your tongues. He said: You spoil with reminders what you have done to ease your affairs; a generous person does not boast when he gives.

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The closest equivalent in English is:

Do a good deed and forget it.

Do good and then forget it.

Also:

Never remind people of the favors you did for them.

And a well-known proverb, very close in spirit, is:

Cast your bread upon the waters.

That is, do good without expecting anything in return or being reminded of it.

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قال بعض حكماء العرب لبنيه. قال أبو عبيد: أراد حتى لا يقع في أنفسكم الطول على الناس بالقلوب ولا تذكروها بالألسنة. قال:
أفسدت بالمن ما أصلحت من يسر*****ليس الكريم إذا أسدى بـمـنـان 

 

He is a seasoned molars ------ إنه لمنجذ

 He is a seasoned molars. The word is derived from the word for the molar, the furthest tooth in a person. This is the opinion of some, but the correct view is that it refers to all the teeth, as mentioned in the hadith. He laughed until his molars were visible. Ash-Shammakh said: "Their molars are like sharp, cutting teeth." It is also narrated that it is "munajid," with a "dal" (د), not a dot, from "najd," meaning a high place, or from "najdah," meaning courage, meaning he is strengthened by experience.

The closest equivalent in English is:

He is battle-hardened.

Meaning, he has been tempered by experience and hardship.

Or:

He is seasoned.

Meaning, he is an experienced expert.

And for the closest rhetorical meaning:

He has cut his teeth on life.

His experience has been sharpened by the trials of life.

(This is a beautiful English expression with a metaphorical connection to teeth, as in the origin of the word "molars").
also
An old hand.

A man with a long history of experience/experience. 

 أي محنك واصله من الناجذ، وهو أقصى أسنان الإنسان. هذا قول بعضهم والصحيح، إنها الأسنان كلها لما جاء في الحديث. فضحك حتى بدت نواجذه.: قال الشماخ

نواجذهن كالحدا الوقيع

ويروى أنه، لمنجد، بالدال، غير معجمة من النجد وهو المكان المرتفع أو من النجدة وهي الشجاعة أي إنه مقوى بالتجارب.

Eating and Criticizing. ---- أكلا وذماً

They eat and they criticize. That is, they eat and they criticize. This proverb is used to describe someone who criticizes something that they might benefit from, even though it doesn't deserve the criticism.

 AI said 

The closest equivalent in English is:

Biting the hand that feeds you.

This is the most common equivalent in terms of meaning.

Another example is:

To eat a man's bread and curse him.

This is closer to the spirit of the old Arabic expression.

A similar, shorter version is:

Complaining while benefiting. 

 

أي يؤكل أكلاً ويذم ذماً. يضرب، لمن يذم شيئاً قد ينتفع به وهو لا يستحق الذم.

Women are the sisters of nations. ---- إن النساء شقائق الأقوام

 The word “shaqa’iq” is the plural of “shaqiqa,” which means anything that is split in two. By “al-aqwam,” he meant men, according to those who say: “al-qawm” applies to men, not women. The meaning of the proverb is that women are the opposite of men and are split from them, so they have the same rights as men.

ِAI said

A similar English equivalent is:

Women are the counterparts of men.

Or, more accurately expressed as a proverb or maxim:

Behind every great nation are its women.

For a broader social meaning:

Men and women are two halves of society.

However, a more direct and insightful translation of the proverb is:

Women are the sisters/equals of men.

 

الشقائق، جمع شقيقة، وهي كل ما يشق باثنين، وأراد بالأقوام، الرجال، على قول من يقول: القوم يقع على الرجال دون النساء ومعنى المثل، أن النساء مخثل الرجال وشقت منهم فلهن مثل ما عليهن من الحقوق. 

If fortune turns its back on a people, their enemy will suffice. --- إذا أدبر الدهر عن قوم كفى عدوهم

 That is, if he helps them, he will be able to deal with their enemy.

AI said  

There are several English proverbs that can be used to express this meaning, the closest being:

When fortune turns against you, even your friends disappear.

Or, a more similar proverb:

Misfortunes never come alone.

And for the meaning of the Arabic proverb—that if time abandons a person, they become too weak for their enemy to exert any effort—there is also:

A falling tree needs no push.

 

أي، إذا ساعدهم كفاهم أمر عدوهم.