Learn Arabic (Modern Standard): البحر والنهر هنا.

This is because of Sun and Moon letters! The letter baa (ب) in البحر is a 'Moon letter,' so we clearly pronounce the 'l' in the definite article ال (al-bahr). But the letter nuun (ن) in النهر is a 'Sun letter.' Sun letters absorb the 'l' sound, so we skip the 'l' and double the 'n' instead, pronouncing it 'an-nahr'.

How do I pronounce والنهر (and the river) when reading out loud?

You pronounce it 'wan-nahr'. The word connects 'and' (و) with 'the river' (النهر). When و attaches to a word starting with ال, the 'a' sound of the ال drops out in speech. Because ن is a Sun letter, the 'l' is also silent. So, you jump straight from the 'wa' to the doubled 'n'.

Why is the word for 'and' (و) attached directly to the word for 'the river' (النهر)?

In Arabic, short one-letter conjunctions or prepositions like 'and' (و) are never written as standalone words with spaces around them. They must physically attach to the beginning of the word that follows them.

The English translation says 'are here', but where is the Arabic word for 'are'?

Arabic does not use a present tense verb for 'is' or 'are'. You simply say the subjects (البحر والنهر, the sea and the river) and follow them directly with the location (هنا, here). This structure is known as a nominal sentence.