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
We don't appreciate what we have but we cry when we lose it.
A wolf is fed by his legs
What goes around comes around.
The quieter you go, the further you'll get.
Put your trust in God, but don't make mistakes yourself
Where it is thin, it breaks.
Get your sleigh ready in summer and your (horse-drawn) cart in winter.
The morning is wiser than the evening.