The conjunction "И" [i] (and) expresses the enumeration of things, people, events, skills, qualities, features etc. The conjunction "И" unites these enumerated objects.
For example:
У меня есть сестра и два брата - I have a sister and two brothers.
На столе лежат книги и тетради - There are books and copybooks on the desk.
По утрам я люблю бегать и принимать контрастный душ - I like running in the morning and take a contrast shower.
Вы говорите по-английски и по-русски - You speak English and Russian.
The conjunction "А" [a] (and/but) expresses contrasting. In the first clause we give information and in the second one (after the conjunction "a") - show what has changed in comparison with this information.
For example:
У меня есть брат, а у тебя сестра - I have a brother and you have a sister.
Сегодня я работаю, а завтра буду отдыхать - Today I work but tomorrow I will take a rest.
Я люблю яблоки, а ты апельсины - I like apples and you like oranges.
Мы лучше знаем английский язык, а не русский - We know the English language well but not Russian.
Sometimes foreigners mix up the usage of conjunctions and use the conjunction "И" instead of "А". For example, "У меня есть брат, и у тебя есть сестра" [U min'ya est' brat, i u tib'ya est' sistra]. If we look at the expression, we will see two facts: "у меня - брат" (I have a brother), and "у тебя - сестра" (and you have a sister).
In combination with the word "также" [takzh'e] the conjunction "А" [a] has almost the same meaning as the conjunction "И" [i].
For example:
В лесу можно увидеть белку и услышать пение птиц - You can see a squirrel and hear bird songs in the wood.
В лесу можно увидеть белку, а также услышать пение птиц - You can see a squirrel and also hear bird songs in the wood.
We can replace the conjunction "И" [i] (and) with "А также" [a takzh'e] (and also) in all abovementioned examples in the beginning of this article.
We often place "А" (and) in the beginning of a sentence - this conveys colloquial connotation to the speech. But it's not always necessary to place this conjunction.
For example:
Что нам делать? = А что нам делать? - What should we do? = And what should we do?
- Возьми зонт - Take the umbrella.
- А зачем? На улице солнце - And for what? It's sunny outside.