Reasoning through the if-clauses:

Read the following information on Alberto, Bernabé, and Carlos.

Alberto, Bernabé and Carlos work in an office building downtown. Each of them has some "vacation hours" saved up and since they are in a lull time at their job, they are essentially free to go and come as they like. All three of them like to go to the beach whenever they get the chance. In fact, they can hardly keep their mind on anything else.

Alberto's office and Carlos's have windows; Bernabé's is an interior office with no window.

This morning, they all heard the weather report: 50% chance of rain all day. It is 11:00 a.m. and it is raining.

Alberto and Bernabé are in their offices working; Carlos is driving to an appointment across town. All three are thinking about going to the beach, as usual. Alberto looks out the window and sees that it is raining. Bernabé doesn't know if it is raining or not, but he knows there is a probability of rain. Carlos is driving in the rain. Which one(s) is(are) thinking what?

A. If it weren't raining, I would go to the beach for lunch.

B. If it isn't raining late this afternoon, I will go to the beach.

C. If it isn't raining, I'll eat lunch at a restaurant on the beach.

A. Si no estuviera lloviendo, iría a la playa ahora.

B. Si no llueve esta tarde, iré a la playa.

C. Si no está lloviendo, voy a almorzar en un restaurante de la playa.

A is a contrary-to-fact sentence: Basic meaning: It is raining, therefore, I am not going to the beach.

B is a sentence expressing a future possibilty: I don't know if it will rain or not this afternoon. There is a possibilty that it will rain, but there is a possibility that it will not rain. If it does not rain, I will go to the beach.

C is similar to B (possibility: I don't know if it is raining now or not. Based on what I know, there is a possibility that it is raining; there is also a possibility that it is not raining. If it is not raining, I will go to the beach.

Sentences like A require imperfect subjunctive or pluperfect subjunctive in the if clause AND conditional in the main clause.

Sentences like B and C require present indicative in the if clause, and future (or present) indicative in the main clause.

To "convert" type A sentences to "past", simply move a step back into the past (using compound teneses: haber + past participle) maintaining the concept of "subjunctive" and "conditional indicative" (pluperfect subjunctive + conditional perfect indicative)

A. Si no hubiera estado lloviendo, habría ido a la playa.