I left home earlier and took the first train because I had work to do the first thing in the morning.
As I had work to do the first thing in the morning, I left home earlier and took the first train.
The work to do the first thing in the morning forced me to leave home earlier and take the first train.
"早朝からやるべき仕事があり早く家を出て始発電車に乗った"
やるべき仕事 work to do
早朝から 朝一番で the first thing in the morning
早く家を出て（いつもより）leave home earlier
始発電車に乗る take the first train
理由を述べる接続詞 because を使って
I left home earlier and took the first train because I had work to do the first thing in the morning.
理由を述べる接続詞 as を使って
As I had work to do the first thing in the morning, I left home earlier and took the first train.
無理やり行きたくもないのに行かされたと強調して表現すると、
無理やり～させる force one to ~
主語を朝一の仕事とすると、
The work to do the first thing in the morning forced me to leave home earlier and take the first train.
I had work to do early in the morning, so I left home early and took the first train.
"I had work to do early in the morning, so I left home early and took the first train."
"I had to start work early, so I left home early and caught the first train."
"I had something I needed to do early in the morning, so I took the first train."
"I had work to do from early in the morning, so I got out of the house early and took the first train."
自然でそのまま伝えやすいのは
I had work to do early in the morning, so I left home early and took the first train.
です。
意味は
「早朝からやるべき仕事があったので、早く家を出て始発電車に乗りました。」
です。
日本 
アメリカ合衆国 
