Learn Turkish: Lütfen ağlama. - Please do not cry.

The verb root is ağla (cry). To tell someone not to do it, we add the negative suffix -ma, creating ağlama (do not cry). We use -ma rather than -me to match the 'a' vowel in the root, following a-type vowel harmony.

Where should I put the emphasis when pronouncing ağlama?

In Turkish negative commands, the spoken stress usually falls on the syllable right before the negative suffix. For ağlama, you emphasize the first syllable: AĞ-la-ma.

Why isn't the word for 'you' (sen) included in this sentence?

Commands are naturally directed at the person you are talking to. The grammatical form of ağlama already implies a singular, informal 'you'. Adding sen is not needed unless you want to sound very emphatic.

Does adding lütfen make this a formal sentence?

Not grammatically. Lütfen (please) makes the tone kinder and softer, but the verb ağlama is still in the informal 'you' form. You would typically use this combination with a friend, family member, or child.