O adubo está no saco.

Because o is the masculine singular definite article, meaning the.

So o adubo = the fertilizer.

In Portuguese, articles are used very often, sometimes more often than in English. Here, the sentence is talking about a specific fertilizer, not fertilizer in general.

• o adubo = the fertilizer
• um adubo = a fertilizer / one fertilizer
• just adubo can appear in some contexts, but it would sound less natural here

Also, o tells you that adubo is a masculine singular noun.

You know it from the article o.

In Portuguese, nouns have grammatical gender, so the article agrees with the noun:

• o adubo = masculine singular
• if it were feminine, it would be a ...

There is no reliable rule that all words ending in -o are masculine, but many are. In this sentence, the article makes it clear.

Portuguese has two verbs that both can mean to be:

• ser → for identity, permanent characteristics, origin, time, etc.
• estar → for location, condition, temporary state

Here the sentence is about location: the fertilizer is in the sack. For location, Portuguese normally uses estar.

So:

• O adubo está no saco. = The fertilizer is in the sack.

Using é here would be wrong.

Está is the 3rd person singular form of the verb estar in the present tense.

It can mean:

• is
• is located
• is situated

In this sentence, it means is in the sense of is located.

The full present tense of estar is:

• eu estou = I am
• tu estás = you are
• ele/ela/você está = he/she/you are
• nós estamos = we are
• vós estais = you are (old-fashioned / rare)
• eles/elas/vocês estão = they/you are

No means in the.

It is a contraction of:

• em = in
• o = the

So:

• em + o = no

That is why:

• no saco = in the sack

Other common contractions are:

• na = em + a = in the
• nos = em + os = in the
• nas = em + as = in the

This kind of contraction is extremely common in Portuguese.

Saco usually means a sack, bag, or large bag, depending on context.

In a sentence about fertilizer, saco is very natural because fertilizer is often kept in a sack or bag. So no saco suggests something like:

• in the sack
• in the bag
• in the fertilizer bag

A different word, like bolsa, would usually refer to a handbag, purse, or bag of a different type, so it would not fit as well here.

Usually, yes, but the exact translation depends on context.

Adubo commonly refers to:

• fertilizer
• sometimes manure
• more generally, a substance used to help plants grow

So if you are reading a gardening or farming context, fertilizer is the safest translation.

The accent shows the stressed syllable and helps distinguish the correct written form.

• está is pronounced with stress on the last syllable: es-TÁ

In modern Portuguese spelling, the accent is part of the standard written form of this verb form, so you must write está, not esta.

Be careful because esta without the accent is a different word, usually meaning this in the feminine form.

Yes, but O adubo está no saco is the most neutral and natural order.

This is the normal pattern:

• subject + verb + location

So:

• O adubo = subject
• está = verb
• no saco = location

You could also say:

• No saco está o adubo.

This is grammatically possible, but it sounds more marked or emphatic, as if you are focusing on where it is or contrasting it with something else.

For a learner, the basic order O adubo está no saco is the best one to use.

You need to change the article, noun, verb, and possibly the location phrase if the noun becomes plural.

Examples:

• Os adubos estão no saco. = The fertilizers are in the sack.
• O adubo está nos sacos. = The fertilizer is in the sacks.
• Os adubos estão nos sacos. = The fertilizers are in the sacks.

Notice:

• o → os
• está → estão
• no → nos when saco becomes plural with article os

In European Portuguese, the pronunciation is more reduced than many learners expect.

A rough guide is:

• O adubo está no saco
• approximately: u ah-DU-bu sh-TA nu SA-ku

A few useful notes:

• o often sounds like u
• de/te sounds can be softer or reduced depending on context
• está often sounds close to shtá in connected European Portuguese speech
• unstressed vowels are often reduced

So in natural European Portuguese, the whole sentence may sound more compressed than its spelling suggests.

Yes.

• O adubo está no saco. = The fertilizer is in the sack.
• O adubo está dentro do saco. = The fertilizer is inside the sack.

The version with dentro de is more explicit. It emphasizes that it is inside the sack, not just associated with it.

Also note the contraction:

• de + o = do

So:

• dentro do saco = inside the sack

Both are correct, but no saco is simpler and more neutral.