ものだから is a strong phrase that is used when explaining something or giving an excuse for one’s reasoning. This grammar structure can simply be thought of as an extension of から ‘because’, with the noun もの ‘thing’ having its usual role of emphasizing what is said before it. Common translations include ‘because (A)’, and ‘the reason is (A)’.
ものだから may appear directly after verbs and い-Adjectives, or な-Adjectives and nouns that are followed with な.

• すみません、今（いま）仕事（しごと）をしているものだから、また後（あと）でかけ直（なお）します。
  Sorry, because I am working right now, I will call you back later.
• 週末（しゅうまつ）は人（ひと）が多（おお）いものだから、週末（しゅうまつ）に行（い）くのはあまりお勧（すす）めしません。
  I would not recommend going on the weekends, because it is usually crowded on weekends.
• 生（なま）の魚（さかな）は苦手（にがて）なものだから、寿司（すし）は食（た）べられない。
  I can't eat sushi because I don't like raw fish.
• 彼（かれ）は新人（しんじん）なものだから、まだこの会社（かいしゃ）のルールをよくわかっていない。
  Because he is new, he doesn't really understand the rules of this company.

Occasionally, ものだから will be abbreviated to もので, or もんで, without any change in the overall meaning. The change of だ to its conjunctive form で indicates that there is something coming after, despite the fact that what comes after may sometimes be omitted if it is obvious.

• 今日（きょう）は夕方（ゆうがた）から娘（むすめ）の発表会（はっぴょうかい）があるもので、私（わたし）はそろそろ帰（かえ）りますね。
  Because my daughter has a recital this evening, I am going to go home soon.
• 最近（さいきん）は日本語（にほんご）を話（はな）す機会（きかい）が減（へ）ってきているもんで、日本語（にほんご）を忘（わす）れてきた感（かん）じがする。
  I feel as if I have forgotten some Japanese, because I have fewer opportunities to speak Japanese these days.
• まあ、安（やす）いもんで...
  Well, because it’s cheap…