ものだから

ものだから gives a reason as an excuse: "the thing is…", "it's just that…". The speaker explains why something happened, usually something that went wrong, and asks the listener to understand. It goes after the plain form; a な-adjective or a noun takes な where だ would be.

A noun or a な-adjective in the present takes な, not だ: ×病気だものだから → びょうき病気なものだから.

ものだから, から and ので

から and ので state a reason. ものだから adds that the speaker could not help it, so it fits late arrivals, mistakes and things you did not mean to do.

Because it explains what already happened, the second half cannot be a request, an order or something you intend to do. Those take から or ので.

• × 寒いものだから、窓を閉めてください → 寒いので、窓を閉めてください

Polite and casual forms

To a superior or a customer, use ものですから. Among friends it shortens to もんだから, or もんで.