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
Оne sees the mote in another's eye and ignores the log in his own.
Being a guest is good, but being at home is better.
The morning is wiser than the evening.
Water wears away a stone.
Poor people are inventive.
Without labor, you can not even catch a fish in the pond.
Crowded, but not aggrieved.
A titmouse in hands is better than a crane in the sky.