Tom failed the exam. He was lazy 

=> As Tomwas lazy, he failed the exam.

- As/ Although/ Though / Even though + S + V ,S + V

= Desipte+ Ving / N / N-ph, S + V

My mother was tired. She had to cook dinner for the family

=> My mother was tired. However, she had to cook dinner for the family

- S + V. However, S + V