Learn Russian: Ноябрь — не тёплый месяц.

In Russian, the present tense of the verb "to be" (am, is, are) is usually skipped. When you connect a subject to a description without a verb, you often use a long dash in its place. Here, Ноябрь — simply means "November is".

How do we know the gender of Ноябрь since it ends in a soft sign (-ь)?

Nouns ending in a soft sign (-ь) can be either masculine or feminine, which can be tricky for beginners! However, there is a handy rule for months: all Russian months end in either -ь or -й, and they are all masculine.

Why does the adjective тёплый (warm) end in -ый?

Russian adjectives must change their endings to match the gender of the noun they describe. Because месяц (month) is a masculine noun, the adjective takes the standard masculine dictionary ending -ый.

Why do we use не here instead of нет?

Нет is used to answer "no" to a question, or to state that something does not exist. Не translates to "not", and is used to negate a specific word or phrase. It is placed exactly before the concept it negates, which is why it comes right before тёплый месяц.