Learn Farsi: بیا با هم روی این مشکل متمرکز شویم.

This is the standard way to say let's in Farsi. You use the imperative of the verb to come directed at your listener, which is biyâ for one informal person, followed by the main action in the first-person plural (we) present subjunctive. You are essentially saying: Come, that we may focus.

If I am talking to a group of friends, does biyâ change?

Yes. Because the proposal is directed at your listeners, you must match the command to them. If you are suggesting this to multiple people, or to someone you address formally, you would change biyâ to the plural imperative biyâvid or its spoken form biyâyn. The main verb stays as shavim because the action is still for we.

I learned that the present subjunctive usually takes a be- prefix. Why is it missing from motamarkez shavim?

The verb to focus is the compound verb motamarkez shodan. For compound verbs that use shodan as their helping verb, the subjunctive be- prefix is very often dropped in modern Farsi. While motamarkez beshavim is technically correct, motamarkez shavim is much more natural and common.

Why do we use the word ru-ye before the problem?

The word ru-ye translates to on or upon. Just like in English, the Farsi verb for focusing, which is motamarkez shodan, pairs with the preposition on to indicate what you are directing your attention toward.