Your confusion is understandable, as the distinction between tenses in Spanish can sometimes seem counterintuitive, especially when comparing them to their English counterparts.

In your example "Hemos pedido al alcalde que ponga más farolas en las calles", the main clause is in the present perfect tense ("Hemos pedido"), and the subordinate clause is in the present subjunctive ("que ponga").

In Spanish, the present perfect tense (e.g., "hemos pedido") is used to describe actions that, although started in the past, have relevance or continuity in the present. It's considered a present tense in terms of its implications for action or state. Hence, it is compatible with the present subjunctive in the subordinate clause when the main clause expresses a wish, a request, an emotion, or a demand – all of which can have present or future implications.

In English, the present perfect can have a slightly different nuance, often emphasizing the completion of an action in the past with relevance to the present. However, in Spanish, the focus is more on the present implications of past actions.

So, in your example, even though "hemos pedido" (we have asked) refers to an action initiated in the past, its effects are considered current – you're currently in a state of having asked. That's why "ponga" (present subjunctive) is used instead of "pusiera" (imperfect subjunctive).

In summary, when the main verb is in the present perfect in Spanish, it usually requires the use of the present subjunctive in the subordinate clause if the sentence structure is such that a subjunctive is appropriate (e.g., expressions of desire, requests, emotions). This is because the present perfect in Spanish is treated as having a present implication, despite its past reference.