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
A holy place is never empty.
A titmouse in hands is better than a crane in the sky.
Without labor, you can not even catch a fish in the pond.
If I knew where I would fall, I would lay some straw there.
Better to see once than hear a hundred times.
You can't command to the heart.
~ Seven babysitters make the kid to lose an eye.
Measure seven times, cut one.