Tanto o arroz quanto o feijão estão na mesma prateleira.

Here, tanto ... quanto works like a pair and means both ... and.

So Tanto o arroz quanto o feijão means both the rice and the beans.

A useful thing to know is that tanto ... quanto can also mean as much ... as in other contexts, but in this sentence it is clearly the both ... and pattern.

O is the masculine singular definite article, like the in English.

In Portuguese, it is very common to use articles with nouns where English would often leave them out. So o arroz and o feijão sound natural here.

Also, both arroz and feijão are masculine nouns, so they take o.

Because the subject is made of two parts: o arroz + o feijão.

Even though each noun is singular by itself, together they form a compound subject, so the verb goes in the plural:

• O arroz está...
• O feijão está...
• O arroz e o feijão estão...

That is why the sentence uses estão, not está.

Yes. That is perfectly natural and probably the most straightforward way to say it.

Using tanto ... quanto adds a little emphasis, something like both the rice and the beans rather than just the rice and the beans. It can sound slightly more careful or emphatic, but it is still normal Portuguese.

Na is a contraction of em + a.

• em = in / on / at
• a prateleira = the shelf

So:

• em a prateleira → na prateleira

This kind of contraction is extremely common in Portuguese.

Because mesmo/mesma has to agree with the noun it describes.

Prateleira is feminine singular, so the adjective must also be feminine singular:

• mesma prateleira

Compare:

• o mesmo livro
• a mesma prateleira
• os mesmos livros
• as mesmas prateleiras

Prateleira means shelf.

That is a little different from estante, which usually means a bookcase, shelving unit, or stand. So na mesma prateleira means they are on the same individual shelf, not just somewhere in the same cabinet or shelving structure.

Portuguese often uses estar to talk about location.

So when you say where something is, estar is usually the normal verb:

• O livro está na mesa.
• As chaves estão na gaveta.

In this sentence, estão na mesma prateleira means the rice and the beans are located there.

No. It can connect other things too, as long as the structure stays parallel.

For example:

• Tanto João quanto Maria trabalham aqui.
• O filme é tanto engraçado quanto emocionante.

In your sentence, it connects two noun phrases: o arroz and o feijão.

The ending ão is nasal, which is one of the sounds English speakers often need to practice.

In Brazilian Portuguese:

• feijão sounds roughly like fay-ZHAUNG with a nasal ending
• estão sounds roughly like es-TAUNG with the same nasal ending

The exact sound is not a normal English ow sound. Air flows through the nose too. Listening and repeating native audio helps a lot with this pattern.