This is a regular verb. It follows the conjugation pattern of the verb falar.
Present:
A Ana mancha a camisa dela com tinta.
A Ana mancha a sua camisa com tinta.
Ana stains her shirt with paint.
Preterite:
O Miguel manchou o sofá com café.
Miguel stained the sofa with coffee.
Conditional:
A Beatriz não mancharia a mesa de propósito.
Beatriz would not stain the table on purpose.