Learn Russian: Давайте начнём собирать букет: я возьму три красные розы. - Let's start putting together the bouquet: I will take three red roses.

This is the special rule for feminine nouns with the numbers 2, 3, and 4. While the feminine noun goes into the genitive singular (розы), any modifying adjective goes into the nominative plural (or accusative plural for inanimate objects, which looks the same). So we get три красные розы. If the noun were masculine or neuter, the adjective would typically be in the genitive plural instead.

Why does начнём (perfective) pair with собирать (imperfective)? Shouldn't they both be perfective since we are talking about a specific single event?

In Russian, phase verbs that indicate the beginning, continuation, or end of an action (like начать, продолжать, закончить) must always be followed by an imperfective infinitive. Even though starting the bouquet is a one-time perfective action (начнём), the process of putting it together itself is viewed as an ongoing action (собирать).

Why is Давайте followed by the 'we' form начнём instead of an infinitive here?

To say 'Let's do something' with a perfective verb, you use Давайте followed by the first-person plural future form of the verb. So, Давайте начнём translates to 'Let's start'. You only use the infinitive after Давайте when you are pairing it with an imperfective verb.

I usually translate собирать as 'to collect' or 'to gather'. Does it mean something different here with ?