"Talk to the doctor."
Translation:Поговори с врачом.
If it's an action to be repeated, the imperfective is fine: "говори с врачом каждый месяц." Duolingo should probably accept both forms, because such a scenario is not out of the question. However, the по- prefix also gives it the sense of "for a little while" which sounds much more natural in the context of a doctor visit.
The rules for using imperfect and perfect verbs are unfortunately more complex than this. In general: the imperative of perfect verbessern is more polite. As an example: in war movies Russian soldiers always shout "давай, давай, давай" and it does not mean "do it again and again". It is just rude.
Давай-давай simply means "Come on!" in a situation where you are impatient to get things done. A single Давай(те) means "Let's" and it's not rude at all. But you are right in suggesting that the imperfective imperative is used to show impatience, unless you refer to a repeated action. Thus, "Говори со мной!" implies "Do talk! Don't keep silent!" No wonder that the imperfective imperative is preferable in giving orders.
Not always. 'С' is replaced with 'со' only before a limited number of nouns/pronouns which start with two consonsonats, including all cases where the first of the two consonans is "с", "з", "ж" or "ш" Remember: со мной, со всем, со всеми, со вторника, со среды, со рта, со лба, со сна, со сном, со льда, со льдом. Also со звездой, со знанием, со знаком, со сказкой, со свечой, со смазкой, со скалкой, со страхом, со страху, со спины, со спиной, со скоростью, со скрипом, со стола, со шкафа, со стула, со стулом etc.
Only поговорите is used in talking to more than one person. In talking to one person, поговори is more informal and can only be used with a peer who you know well, a friend, a close relative or a child. It is not recommended in talking to an adult stranger (may be considered impolite).
The two Russian imperative forms of поговорить are поговори (informal singular) and поговорите. The plural form which is also used as a polite or formal singular form is the same for the imperative and the future: поговорите (= talk / you will talk). Поговоришь only means “you will talk” (informal singular: “thou willst talk” if you like).
Here, the 2nd person plural form just happened to be the same as the 2nd person plural imperative. The singular imperative is different (говорить→говори, читать→читай, писать→пиши)
Even the "plural you" form is not always the same as the imperative:
- любить → лю́бите / люби́те
- читать → читаете / читайте
- жить → живёте / живи́те
- петь → поёте / пойте
- стоять → стоите / стойте
Basically, it is the same for some И-conjugation verbs with fixed stress.