Adubei o canteiro ontem.

Adubei is the 1st person singular form of adubar in the pretérito perfeito simples (simple preterite).

That means it expresses a completed action in the past: something the speaker did and finished.

• adubo = I fertilize / I do fertilize
• adubei = I fertilized

In this sentence, it tells you that the action happened yesterday and is viewed as finished.

In Portuguese, the subject pronoun is often left out because the verb ending already shows who the subject is.

Here, adubei clearly means I fertilized, so eu is not necessary.

• Adubei o canteiro ontem. = natural, neutral
• Eu adubei o canteiro ontem. = also correct, but eu adds emphasis, contrast, or clarity

For example, you might use eu if you want to contrast with someone else:

• Eu adubei o canteiro, não foi o João.

The infinitive is adubar.

This is a regular -ar verb. Its simple preterite pattern is regular:

• eu adubei
• tu adubaste
• ele/ela adubou
• nós adubámos
• vós adubastes
• eles/elas adubaram

In European Portuguese, nós adubámos has the written accent to distinguish it from the present form adubamos.

Portuguese uses the definite article more often than English.

So where English might sometimes say I fertilized the garden bed or just I fertilized a garden bed depending on context, Portuguese often naturally says o canteiro when the speaker has a specific bed in mind.

• o canteiro = the bed / flowerbed / garden bed

If you removed the article, the sentence would usually sound less natural in ordinary speech unless there were a special context.

Here, canteiro means a garden bed, flowerbed, or planted plot.

The exact English word depends on context:

• if it has flowers: flowerbed
• if it has vegetables or other plants: garden bed or plot

So canteiro is the planted area itself, not the whole garden.

Yes, canteiro is masculine singular, which is why it takes o:

• o canteiro = the garden bed
• os canteiros = the garden beds

You can often guess gender from the ending, and many nouns ending in -o are masculine, though there are exceptions in Portuguese.

Yes, ontem can move.

The given sentence has a very natural, neutral order:

• Adubei o canteiro ontem.

But these are also possible:

• Ontem adubei o canteiro.
• Adubei ontem o canteiro.

The difference is mostly one of focus or style:

• Ontem adubei o canteiro gives a little more prominence to yesterday
• Adubei o canteiro ontem is a very common neutral order

Because the pretérito perfeito simples normally presents the action as completed.

So adubei means the speaker did it and the action is finished.

If you wanted an ongoing, habitual, or background past action, Portuguese would normally use a different form, such as the imperfect:

• Adubava o canteiro... = I used to fertilize / I was fertilizing the bed...

With ontem, adubei is especially natural because it fits a finished action at a specific past time.

Yes.

• adubo = fertilizer / manure / compost, depending on context
• adubar = to fertilize / to manure

So adubei literally comes from the verb built from adubo.

This is useful vocabulary to remember as a family:

• adubo = the substance
• adubar = the action

No, it is completely correct.

It just sounds a bit more explicit or emphatic than the version without eu.

In Portuguese, especially in ordinary speech, the pronoun is often omitted unless there is a reason to include it:

• emphasis
• contrast
• clarification
• avoiding ambiguity

So:

• Adubei o canteiro ontem. = natural and neutral
• Eu adubei o canteiro ontem. = also correct, with extra emphasis on I

Yes. Depending on context, adubei could be translated as:

• I fertilized
• I manured
• I put fertilizer on
• I fed the bed/soil

The best translation depends on what kind of adubo was used and on how natural you want the English to sound.

So even though the Portuguese form stays the same, English may choose different wording.