The Spanish word 'si' meaning 'if' or 'whether' comes directly from the Latin word 'si', which had the same meaning. This is one of the most stable words in the Romance languages, maintaining both its form and meaning virtually unchanged from Latin to modern Spanish.

Since 'si' is already one of the most basic and common words in Spanish, there aren't really any simpler related Spanish words to connect it to. It's one of the first words Spanish students learn when studying conditional sentences and indirect questions.

While English 'if' is not related to Spanish 'si', there is an interesting parallel in how both languages use their words for 'if'. Both Spanish 'si' and English 'if' are used to form conditional sentences ('Si llueve, me quedo en casa' = 'If it rains, I'll stay home') and indirect questions ('No sé si viene' = 'I don't know if he's coming').