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
With a darling, even a hut is a paradise.
Live for a century, learn for a century, but die being a fool anyway.
Where it is thin, it breaks.
Don't go to the woods if afraid of wolves.
Put your trust in God, but don't make mistakes yourself
If you are chasing two hares at once, you won’t catch a single one.
Who is talking about what, and a lousy one about the bath.
Fear has big eyes.