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
An old friend is better than two new ones.
Poor people are inventive.
Another's soul is darkness.
They do not seek good from good.
There are no friends for taste and color.
We don't appreciate what we have but we cry when we lose it.
Hurry to much and you will make people laugh.
You can't spoil the porridge with butter.