Reguei o canteiro cedo.

Because Portuguese often drops the subject pronoun when the verb ending already makes the subject clear.

• Reguei means I watered
• The ending -ei shows it is first person singular in the past

So Reguei o canteiro cedo is perfectly natural, and adding eu is only needed for emphasis or contrast:

• Eu reguei o canteiro cedo, não ele. = I watered the flowerbed early, not him.

Reguei is the pretérito perfeito simples in Portuguese, usually called the simple preterite in English explanations.

It shows a completed action in the past:

• Hoje rego o canteiro. = I water / am watering the flowerbed today.
• Ontem reguei o canteiro. = I watered the flowerbed yesterday.

In this sentence, it means the watering happened as a finished event.

This is a spelling change used to keep the correct sound of g.

The verb is regar. In Portuguese:

• g before a, o, u usually has a hard g sound
• g before e or i would normally sound different

To keep the hard sound before e, Portuguese inserts u:

• regar → reguei

So the u is there mainly for spelling/pronunciation reasons.

Yes, regar is basically a regular -ar verb, but with that normal spelling adjustment in forms before e.

For example:

• eu reguei
• tu regaste
• ele/ela regou
• nós regámos
• vocês regaram

So it behaves like a regular -ar verb in the preterite, except for the spelling in reguei.

O canteiro is the direct object of the verb reguei.

It answers the question:

• What did I water?
• O canteiro

So the structure is:

• Reguei = verb
• o canteiro = direct object
• cedo = adverb

Because Portuguese often uses the definite article where English may or may not use the.

Here, o canteiro means the flowerbed / the garden bed and refers to a specific one, not just any flowerbed in general.

If you removed the article and said simply reguei canteiro, that would sound ungrammatical here.

Cedo is an adverb. It means early.

It modifies the action reguei, telling us when the action happened.

So:

• Reguei o canteiro cedo = I watered the flowerbed early.

That is a very natural position for an adverb of time in Portuguese.

The order here is:

• verb
• object
• time adverb

So Reguei o canteiro cedo sounds very normal and neutral.

Other word orders are possible, but they may sound more marked or may shift emphasis:

• Cedo, reguei o canteiro. = more emphasis on early
• Reguei cedo o canteiro. = possible, but less neutral in many contexts

For a learner, the original sentence is the safest and most natural pattern.

Not exactly.

Cedo means early, not specifically in the morning.

So it could mean:

• early in the morning
• early in the day
• simply earlier than expected

If you want to say in the morning, you would use something like:

• de manhã

Compare:

• Reguei o canteiro cedo. = I watered the flowerbed early.
• Reguei o canteiro de manhã. = I watered the flowerbed in the morning.

It can have different meanings depending on context, but in this sentence it most naturally means a flowerbed, garden bed, or planted bed.

A learner should remember that many Portuguese nouns are a bit broader than a single English word, so the exact translation depends on context. Here, because the verb is regar, the gardening meaning is the obvious one.

In European Portuguese, you would normally use a clitic pronoun attached to the verb:

• Reguei-o cedo. = I watered it early.

This is very useful to know because European Portuguese strongly prefers this kind of object pronoun placement in affirmative main clauses.

So:

• Reguei o canteiro cedo.
• Reguei-o cedo.

Because Portuguese verb endings usually carry subject information.

The ending -ei in the preterite tells you it is eu = I.

So even without eu, the form itself already means:

• I watered

This is one of the big differences from English, where the verb usually does not show the subject so clearly.

The sentence itself works in both European and Brazilian Portuguese, but a few learning points are especially helpful for European Portuguese:

• dropping eu is very normal
• using o with canteiro is completely natural
• if you replace the object with a pronoun, European Portuguese strongly prefers Reguei-o rather than a separate pronoun form

So the sentence is not uniquely European Portuguese, but it fits European Portuguese usage very well.

No. That changes the tense and the meaning.

• Reguei o canteiro cedo = I watered the flowerbed early.
  A completed action.

• Regava o canteiro cedo = I used to water / was watering the flowerbed early.
  A habitual, repeated, ongoing, or background action in the past.

So reguei is for a single completed event, while regava gives a different past viewpoint.