How can I distinguish the particle で from the て form of です?

Question

雨で出かけられない。

Can't で in this kind of sentences be both?

雨 + で (particle) + 出かけられない。
Because it's raining, I can't go out.

雨 + で (て form) + 出かけられない。
(雨です。出かけられない。)
It's raining, (and, then) I can't go out.

Answer 1

In this simple sentence, this で is a particle, not the te-form of だ. There is no reason to parse it as a compound sentence that has two main clauses.

To interpret it as the te-form of だ, you need to indicate it belongs to another clause in some way or another. For example,

あいにく今日は雨で、(私は)出かけられない。

Here 雨で clearly belongs to a different clause with a different topic (今日), so this is a compound sentence with two main clauses. (I also added a comma, which is not strictly necessary but works as a hint that there is a "break" after the で.)

When で is the te-form, you should be able to use also でして (te-form of です) and であって (te-form of である) naturally:

あいにく今日は雨でして、出かけられません。
あいにく今日は雨であって、出かけられない。

Related:

• Is the で in というわけで the de-particle or the te-form of だ?

Answer 2

Trying to look at it from a mechanical perspective:

The reason there's potential for ambiguity is that だ (forget です for a moment, because we aren't at the end of the main clause) is a contraction of である, where the で unambiguously is particle-で. So these are separate analyses of the same thing:

猫だ = <猫><である> "[I] am a cat" (loosely)

<猫で><ある> -> "[I] exist as a cat" (ある has a more specific meaning than English "is")

Since で is the te-form of だ, that makes it a contraction of the te-form of である, that is, a contraction of であって.

So the question is really, "is there actually a hidden あって here that has been completely elided?".

There could be, merely following those rules; but it's unlikely. The sentence doesn't need that complexity to make sense. It's somewhat like suggesting that someone who says "It's raining; I can't go out" in English has left off an implicit "Being that" at the beginning (and chosen the wrong punctuation).

Naruto's example makes it more sensible in two ways: by giving additional arguments that would make sense for the implied あって predicate, and by implying an intermediate argument for 出かけられない. (After all, saying "X で Y は [verb]" - intending X and Y both to describe the verb - is a bit awkward.)

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The possibly controversial part: I'm not actually convinced that the て itself of a te-form is distinct from particle-で in the first place. Sometimes it needs to be translated differently to sound naturally in English; but the more I look at it, the more it seems to be performing the same function.

The form results from an 音便 of the い-stem of the verb, and that stem by itself sure looks to me like what I'd call a gerund in English. (Consider some examples that are commonly used as nouns, such as 書き from 書く.) Which is to say, it functions nominally, making it suitable to put in front of particle-で.

Adding te-form で allows it to modify a verb, by making it a gerundive (which can be adverbial) - but the modification is, in my mind, the same sort that particle-で-marked nouns normally make to verbs. Hence, for example, te-iru form:

飛んでいる　"is flying"

<飛んで><いる> "flying (description)" + "exist (animate)"

<飛ん><で><いる> "flight" + connective + "exist (animate)"

(where 飛ん is an 音便 of 飛び, which can be used as is!)

If we take the で as te-form で in the usual interpretation, then it's something like "-ly" in this context, making it possible to modify いる.

If we take it as particle-で, then we might say that the subject "exists as [something which is] flying", or "exists in [a state of] flight".

But are those really different? I don't think they are, and I think it's only the difficulty of explaining Japanese (and English!) in English that makes one suppose otherwise.