で at the end of a word

Question

What impact does the 「で」 add if placed like this:

秋の風は静かで vs. 秋の風は静か

or

心配しないで vs. 心配しない

Answer 1

We have two different words here -- two different で's. Auxiliary verb vs. Particle.

In the phrase 「[秋]{あき}の[風]{かぜ}は[静]{しず}かで」, the 「で」 is the [連用形]{れんようけい} (= "continuative form") of the affirmation auxiliary verb 「だ」.

Thus, the phrase will surely be followed by another phrase in regular prose-style writing. As a title of a creative writing, however, it can be used by itself without another phrase following it.

An example of when the phrase is followed by another would be 「秋の風は静かで、[失恋]{しつれん}した[私]{わたし}の[心]{こころ}を[癒]{いや}してくれる。」 = "The autumn wind is quiet and (therefore) it soothes my broken heart"

「秋の風は静か」 sounds more like a fractured sentence that is "acceptable" in informal conversation but it would require an auxiliary verb or verb phrase such as 「だ」 and 「である」 at the end in any kind of formal writing.

Now, on to the second pair of phrases.

The 「で」 in 「[心配]{しんぱい}しないで」 is completely different. It is a particle attached at the end of an informal imperative.

Both 「心配しないで。」 and 「心配しない。」 can mean "Don't worry!" in informal speech, but the former would sound a little friendlier.