Radicals, unlike in the english language are POSTpoisitions, unlike prepositions, meaning that they modify the word or phrase BEFORE them.
Especially in reading real Japanese, with Kanji, they are invaluable in breaking up phrases into discernible parts. There are many, but you only need to concern yourself with a few when first learning Japanese: wa, ga, o, de, ni, ka, e, ...and the like. Others come with time.