With accent marks:
Дарёному коню́ в зу́бы не смо́трят.
Do not look at a given horse's teeth.
In the old days, when buying horses, people determined the age of a horse by the teeth. Sometimes, having received a horse as a present, some began to complain that they were given a walking antiquity...
This Russian proverb should be understood as follows: never say that you don’t like a gift or that it is bad. This will offend the giver while he really wanted to please you. It is not polite. Be sure to day thank you. And do not forget that not everyone has the opportunity to make expensive gifts.
Other Russian proverbs
There are no friends for taste and color.
Repetition is the mother of learning.
The forbidden fruit is sweet.
The further into the forest, the more firewood is there.
(lit.) Whoever gets up early, God gives him
Hurry to much and you will make people laugh.
Patience and labor will grind everything.
Man does not live thanks to bread alone.