Off the top of my head I'd say we use "есть" when it's important for us to stress the fact that we own/possess something. When something else is of more importance (location, colour, quality...), we often omit it. Compare: - У меня есть квартира. (The fact that I possess it is what's important in my utterance) - У меня (есть) квартира в Москве. (If I omit "есть", my main idea will be to point out that my flat is in Moscow, not somewhere else) - У тебя красивая квартира. (The fact that your flat is nice is given more importance here than the fact that it's in your possession, it would be very unnatural to use "есть" here). That's a really good question, Danny! I'll keep thinking, maybe there's something else to say about it.

In russian language the word "есть" omits when it means "to be" and the sentence is simple present in engish language. I can translate "У тебя хорошая квартира." like: 1. "You have a nice apartment." 2. "Your apartment is nice." The second variant is literal translation for russian. Therefore "есть" (like "is") omits. ------------------------- В русском языке слово "есть" опускается, когда оно сопоставимо с глаголом "to be" в английском и произносится в настоящем времени. На английский язык предложение "У тебя хорошая квартира." можно перевести двумя способами: "You have a good apartment" or "Your apartment is nice". Дословный (логический) вариант для русского языка - будет второй. Поэтому, "есть" (соответствующее "is") - опускается.

To omit when you name something/somebody. - Это ручка (Это есть ручка - It's a pen); - Он учитель (Он есть учитель - He is a teacher). To use when you want to point something's belonging or existing. - У тебя есть ручка. - You've got a pen. - Есть минутка? - Do you have a minute?