Learn Arabic (Modern Standard): نحن نفهم القاعدة دائما.

To make a present tense verb negative, you simply put لا (laa) directly in front of it. 'We do not understand the rule' would be نحن لا نفهم القاعدة (nahnu laa nafhamu al-qaa'ida).

Why does the verb نفهم (nafhamu) start with the letter ن (noon)?

In Arabic, present tense verbs use prefixes to show who is doing the action. The letter ن is the specific prefix for 'we', which perfectly matches the independent pronoun نحن (nahnu).

Do I have to say نحن at the beginning of the sentence?

No, you could just start with the verb and say نفهم القاعدة دائما. Since the verb نفهم already has the 'we' prefix built into it, the pronoun نحن is optional. Including it at the beginning just adds a bit of emphasis.

Why does the word دائما (always) have an extra alif and two dashes at the end?

Those two dashes are a pronunciation marker called 'tanween' that adds an '-an' sound (making the word 'daa'iman'). You will often see this specific ending on Arabic adverbs (words that describe when or how something happens), such as غدا (ghadan - tomorrow) or شكرا (shukran - thank you).