Learn Spanish (Latin America): De mostrar el medidor un problema, la alarma sonará. - If the gauge shows a problem, the alarm will sound.

Using de + an infinitive (like mostrar) is a way to express a condition. It works just like si (if) + a conjugated verb. In this sentence, de mostrar means exactly the same as si muestra ("if it shows"). It is simply a more formal alternative.

Why is the subject el medidor placed after the verb instead of before it?

In the de + infinitive conditional structure, you cannot put the subject between the preposition de and the infinitive. The subject must come immediately after the infinitive. Therefore, you have to say de mostrar el medidor rather than de el medidor mostrar.

Is this "de + infinitive" structure common in everyday conversation?

Not really. This conditional structure is mostly used in formal writing, technical manuals, or legal warnings (which makes sense for a sentence about a gauge and an alarm!). In everyday speech, you would usually just say Si el medidor muestra un problema...

How would this sentence change if I wanted to say "If the gauge had shown a problem" using this same structure?

To express a past hypothetical condition, you would use de + the perfect infinitive (haber + past participle). The phrase would become De haber mostrado el medidor un problema... (If the gauge had shown a problem...).