In English, sentences usually have the form subject-verb-object. Word order determines the meaning. That is how you know that Nakamura-san is doing the seeing and Shimada-san is the person being seen in this sentence:
In Japanese, the normal word order is subject-object-verb. In addition, Japanese uses particles, which are syllables that tell how a word is used. Here is the previous sentence in Hiragana, with the particles in large red type:
Let’s start with two particles, and , that are so important that they deserve their own page.