Learn Arabic (Modern Standard): ما سرقت المحفظة.

To say you didn't do something in the past, you just place ما (ma) directly before the past tense verb. Here, putting ما before سرقت (saraqtu - I stole) changes it to "I did not steal".

How do we know سرقت means "I stole" and not "you stole" or "she stole"?

Without vowel marks (harakat), the written word سرقت can actually mean "I stole" (saraqtu), "you stole" (saraqta or saraqti), or "she stole" (saraqat). In real life, you figure out which one it is from the context of the conversation. Because our English translation says "I", we know it is pronounced saraqtu here.

Where is the word for "I" (أنا) in this sentence?

In Arabic, the person doing the action is already built into the verb. The suffix ـت on سرقت (saraqtu) means "I stole". You can add أنا (ana) at the beginning for emphasis, but it is completely optional and usually left out.

Is the "al" sound pronounced clearly in المحفظة?

Yes! The word for wallet starts with the letter م (meem), which is a "moon letter." This means the definite article ال (al-) is pronounced clearly as "al-mihfadha", without blending the "L" sound into the meem.