ものですから / もので

ものですから and もので give a reason as a polite excuse: "I'm afraid it's because…". You use them to a superior, a customer or a stranger when explaining why something went wrong or why you can't do what was asked. They go after the plain form; a な-adjective or a noun takes な.

The verb stays in the plain form even though the sentence is polite: ×遅れましたもので → おくれた遅れたもので.

もので or ので

ので states a reason politely. もので adds an apology: the speaker could not help it and hopes you will understand. Like any excuse, it explains what has happened; it cannot lead into a request or an instruction.

• × 時間がないもので、急いでください → 時間がないので、急いでください

Leaving the rest unsaid

The sentence can stop at もので and let the listener supply the consequence. This turns a refusal or a request into a hint.

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ものですから is the more formal of the two. もので is lighter and turns up in speech of every level. In casual speech both shorten to もんで.