Questions & Answers about Если троллейбус не придёт, я пойду до метро пешком.
This is a very common difference between English and Russian.
In Russian, with a real future condition, the if-clause can use a future form:
Если троллейбус не придёт, я пойду...
Literally, that first verb is future: if the trolleybus doesn’t come / doesn’t arrive.
English normally uses the present after if, but Russian does not have that same restriction. So this is completely normal Russian.
Because the sentence is about one specific future arrival.
- придёт = perfective future, one completed arrival
- приходит = imperfective present, usually habitual or repeated: it comes / it is coming
- будет приходить = repeated future: it will keep coming / will come regularly
Here the speaker means one event: the trolleybus may or may not arrive. That is why придёт is the natural choice.