Learn Arabic (Modern Standard): الطعام داخل الصندوق.

In Arabic, there is no word for 'is' or 'are' in the present tense. When you put a subject like الطعام (the food) directly before its location, the 'is' is simply understood!

How do we use the word for 'inside' (داخل) here?

داخل (dākhila) is an adverb of place. In Arabic, words that tell you where something is are placed directly before the noun they relate to. You just say 'inside the box' without needing an extra preposition for 'in' or 'at'.

Why doesn't داخل have الـ (al-) attached to it?

In Arabic, adverbs of place like داخل often connect to the next noun using a 'possession' phrase structure (called an Idafa). The rule is that the first word in these pairings never takes الـ (al-). You can think of the literal translation as 'the inside of the box.'

If I read this aloud with the ending vowels, what is the sound at the end of داخل?

It takes a short 'a' sound (fatha), making it dākhila. Adverbs of time and place typically end with this 'a' sound when they are telling us where or when an action or situation is happening.