"Свет" and "свеча" are the same part of speech, they have the same lexical root, and they haven't got any other word parts, only the root and the ending. But "свеча" doesn't look like a form of "свет". Are they really forms of the same word?
No, they're not. Свет means "light" and свеча means "candle".
Etymologically, свеча comes from Proto-Slavic *světja, where j was the instrumental suffix (a suffix which forms the tool names, like "-er" in English "lighter"). It was "something you make light with".
Due to palatalization, this suffix has since changed its form and merged with the root, and is not perceived as a separate morpheme anymore. But it was there.
They are two distinct words. However, etymologically they are related:
From Old East Slavic свѣча (svěča, “candle”), from Proto-Slavic *svěťa (“candle”), from *světъ (“light”) + *-ja.
In modern Russian, the roots are different: "-свет-" and "-свеч-". Even though they come from the same root historically (the English word "white" also comes from this root) does not guarantee they have the same root in modern language.