In the simple past tense, negative and question forms are made using the auxiliary verb "do" (in its past form, "did") followed by the simple form of the main verb. This page explains the rules.

අතීත නැත අර්ථය ගෙන දීමට do සහයක ක්‍රියාපදය එහි අතීත කාලයට did ලෙස හරවා ඊට not එකතු කරනු ලැබේ.
මෙහිදී did not යන්න තිරව (නිල) කීමටත් didn't යන්න තිර නැතිව (නොනිල) කීමටත් යොදා ගනී.

Negatives in the simple present are formed by adding didn't (informal) or did not (formal) before the simple form of the verb.
The verb BE is an exception to this; in the case of BE, we just add n't (informal) or not (formal) after "was" or "were": BE හිදී මෙය වෙනස්ය, එහිදී was හෝ were සමග එකතු කෙරේ.

Simple past statement	Informal negative	Formal negative
I had a car.	I didn't have a car.	I did not have a car.
You ate my toast.	You didn't eat my toast.	You did not eat my toast.
He was here yesterday.	He wasn't here yesterday.	He was not here yesterday.
They were in the park.	They weren't in the park.	They were not in the park.

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