Learn Arabic (Modern Standard): هل النهر الكبير هنا؟

This happens because the first letter of 'river', ن (noon), is a 'Sun letter.' When the definite article ال (al) comes before a Sun letter, the 'l' sound blends into it. Instead of 'al-nahr', we pronounce it 'an-nahr', doubling the ن sound.

Does the 'l' sound disappear in الكبير (the big) as well?

No, it stays! The letter ك (kaaf) is a 'Moon letter.' With Moon letters, the 'l' in ال (al) is pronounced normally. So we say 'al-kabiir', keeping the 'l' sound clear.

Why do both 'river' (النهر) and 'big' (الكبير) have ال attached to them?

In Arabic, adjectives must match the noun they describe. Because we are talking about a specific river (النهر, the river), the adjective must also be made specific by adding ال. Literally, it reads as 'the river the big.'

Where is the Arabic word for 'is' in this sentence?

Arabic does not use a word for 'is' or 'are' in simple present tense sentences. If you just put the words for 'the big river' and 'here' together, the 'is' is naturally understood.

What exactly does هل (hal) do at the beginning?

هل is a question word used to create a simple yes or no question. It works like putting 'Is' or 'Does' at the start of an English sentence. Without it, the sentence is just a statement: 'The big river is here.'