Ты ошибаешься, вчера было восемнадцатое ноября.

Breakdown of Ты ошибаешься, вчера было восемнадцатое ноября.

ты
you (singular)
вчера
yesterday
быть
to be
ошибаться
to be mistaken
ноябрь
November
восемнадцатый
eighteenth

Questions & Answers about Ты ошибаешься, вчера было восемнадцатое ноября.

Why do we use the neuter form восемнадцатое for 'eighteenth'?
When stating the date in Russian, you use the neuter form of the ordinal number. This is because the neuter noun число (number or date) is implied. You are literally saying 'the eighteenth [date] of November', even though the word число is normally left out.
Why is the month written as ноября instead of ноябрь?
Because you are saying 'the eighteenth of November', which requires the Genitive case. Ноябрь is a masculine noun ending in a soft sign (-ь), so to form the Genitive, you drop the soft sign and add -я.
Why is the verb 'was' translated with the neuter form было?
In the past tense, Russian verbs must agree in gender with their subject. The subject here is the neuter date, восемнадцатое [число]. Because the implied subject is neuter, the past tense of the verb 'to be' must also be neuter: было.
I see the -ся ending. Is 'to be wrong' a reflexive verb in Russian?
Exactly! English uses the verb 'to be' plus the adjective 'wrong'. Russian uses the single reflexive verb ошибаться, which literally means 'to make a mistake' or 'to be wrong'. For the ты (you) form, it conjugates as ошибаешься.

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