Thank you and sorry (спасибо, извините)

Thanking

Спасибо is thank you, and it is invariable — no endings, no agreement, nothing to learn beyond the word itself. It comes from the old phrase спаси Бог, "God save you", but nobody hears that any more.

Thank you, I understand.
Спасибо, я понимаю.

Two Words for Sorry

Извините and простите are both polite apologies and both are the вы form, which is what you want with a stranger.

wordstrengthtypical use
извинитеlighterexcuse me, getting attention, small slips
проститеheaviergenuine apology, asking forgiveness

In practice they overlap heavily, and either will serve. Извините is the one to reach for when stopping someone in the street.

Excuse me, I do not understand.
Извините, я не понимаю.
Excuse me, where is the museum here?
Извините, где здесь музей?

Agreeing

Конечно means of course — the plain, warm agreement. Note the spelling trap: чн here is pronounced shn, so it sounds like "ka-NYESH-na". Russian does this in only a handful of words, and this is the commonest of them.

Being Glad

Рад is a SHORT adjective — a small closed group in Russian that works only as a predicate, never in front of a noun. It still agrees: рад (m), рада (f), рады (pl).

Я рад. — said by a man. Я рада. — said by a woman.

Приятно means pleasant, and очень приятно is the fixed phrase you say when meeting someone — "nice to meet you".

Nice to meet you! I am a student.
Очень приятно! Я студент.

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