"The leisurely old man reads a book."
Translation:Senex otiosus librum legit.
When do you know how to place the adjective in the front or back in a Latin sentence?
Do you mean the adjective before or after the noun?
It depends which kind of adjectives. It's a matter of type of adjectives, and also a matter of emphasis.
An adjective can either precede or follow its noun: for example, "a good man" can be both bonus vir or vir bonus. Some kinds of adjectives are more inclined to follow the noun, others to precede, but the precise factors conditioning the variation are not always obvious.