Learn Italian: L'acqua è nel frigorifero. - The water is in the fridge.

In Italian, when the preposition in (meaning 'in' or 'inside') is followed by a definite article like il (the), they combine into one word. Since frigorifero is a masculine noun starting with a consonant, it takes the article il. The combination of in + il makes nel.

Could I just say in il frigorifero?

No, in Italian you must always combine in with the definite article when they appear together. Leaving them separate as in il is grammatically incorrect. You have to use the combined form, nel.

Why is it L'acqua instead of La acqua?

Acqua is a feminine noun that starts with a vowel. When the feminine singular article la comes before a vowel, the a is dropped and replaced with an apostrophe to make it flow better when spoken.

Why does è have an accent mark?

The accent mark shows that è means 'is' (from the verb to be). Without the accent, the word e simply means 'and'. The accent distinguishes the two words and tells you to stress the vowel slightly more when speaking.