Out of boredom, and in order to practice my Japanese-English translation skills a bit, I thought I'd start regularly adding little tidbits from the Space ALC website: frequently asked questions about Japanese grammar, vocab, etc.
Since nobody asked for an answer to these questions, let's call this series "Bueller's Seldom-Asked Questions".
Today's SAQ is about aspect:
http://home.alc.co.jp/db/owa/jpn_npa?stage=2&sn=32
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How are these sentences different?
1) "毎日歯を磨きます"
2) "毎日歯を磨いています"
"毎日歯を磨きます" and "毎日歯を磨いています" both represent a repetitive action, habit, etc. Although it is difficult to sense the difference between the two in this context, this doesn't mean that they have exactly the same meaning.
When only such phrases as "毎日" or "毎朝", are used, the difference between "〜する" and "〜ている" is not apparent. However, the difference between the two becomes clear in the following examples:
これからは毎日歯を磨きます。(OK!)
これからは毎日歯を磨いています。(NO!)
３年前からずっと毎日歯を磨きます。(NO!)
３年前からずっと毎日歯を磨いています。(OK!)
From this, we can say that in the case of "毎日〜している", we know that the habit/trend exists now, but it is not clear if it will continue into the future. When using "毎日〜する", we know that the habit/trend exists now, but it is not clear if it existed in the past.
"毎日〜している" represents something that wecan see happening, whereas "毎日〜する" is more of a statement about general trends than actual occurences. For example, the phrase "アメンボが水の上を走っている" means that "a water strider is [actually now] skimming across the surface of the water", whereas the phrase "アメンボは水の上を走る" means "water striders [have a tendency to] skim across the surface of the water".
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So, I guess, basically, "毎日歯を磨きます" means something between "I brush my teeth every day" and "[I expect that] I will continue to brush my teeth every day". "毎日歯を磨いています" means something between "I brush my teeth every day" and "I have been brushing my teeth every day".
