Well I will try to be concise and to the point here.
The pronoun can be omitted if it is not the subject of the relative clause. And the subject "does something" (in contrast to something being done to it).
I will explain further by way of examples:
Do you know the girl (who/m) he's talking to?
The subject is "he" he is the one that is doing the talking to her so we can´t omit he but "whom" (the relative pronoun - because it´s the object of the action (the conversation).
A simple program that converts an ammount of seconds in ours and minutes.
A simple program is the "doer" here, it is the one that converts, so "that" is not left out.
We are talking a relative clause here, but in another context (with other meaning) it could be left out.
Now let´s say, the sentence "a simple program converst an ammount of seconds in hours and minutes." is grammatically correct but then, as in Portuguese it also leaves the particle "que
So, it´s translation would be "um programa simples converte uma quantidade de segundos em horas e minutos".
O "que" da última tradução não está presente, portanto não se refere à coisa mencionada (anteriormente). E claro, faz uma diferença enorme na tradução. http://www.soportugues.com.br/secoes/morf/morf51.php
Excuse me because I had to make forays on Portuguese grammar here, but it was for clarity´s sake.
To those that want to dig deeper or train on this topic:http://esl.fis.edu/grammar/rules/relative.htmhttps://forum.wordreference.com/threads ... s.2532772/