O aquarista monitora o pH desde hoje de manhã.

Because o means the.

So o aquarista = the aquarist.

In Portuguese, it is very normal to use a definite article before a noun when you are talking about a specific person or thing. Here, the sentence is referring to a particular aquarist, not just any aquarist in general.

• o aquarista = the aquarist
• um aquarista = an aquarist

If you said just aquarista by itself, it would usually sound incomplete in this kind of sentence.

Aquarista can refer to either a man or a woman. The noun itself usually stays the same; the article shows the gender:

• o aquarista = the male aquarist
• a aquarista = the female aquarist

This is common with many Portuguese nouns ending in -ista:

• o turista / a turista
• o dentista / a dentista
• o pianista / a pianista

So in your sentence, o aquarista tells you the aquarist is male.

Monitora is the 3rd person singular present tense of the verb monitorar (to monitor).

Conjugation:

• eu monitoro = I monitor
• você / ele / ela monitora = you / he / she monitors
• nós monitoramos = we monitor
• eles / elas monitoram = they monitor

So here:

• O aquarista monitora = The aquarist monitors / is monitoring

Because in Portuguese, the present tense is often used for actions that started in the past and are still continuing now, especially with time expressions like desde... (since...).

So:

• O aquarista monitora o pH desde hoje de manhã.

can naturally mean:

• The aquarist has been monitoring the pH since this morning.

In English, you usually prefer the present perfect progressive (has been monitoring) for this idea. Portuguese often uses the plain present tense in the same situation.

You could also hear:

• O aquarista está monitorando o pH desde hoje de manhã.

That is possible too in Brazilian Portuguese, but monitora is perfectly natural.

Desde means since or from, depending on context.

In this sentence, it means since:

• desde hoje de manhã = since this morning

It marks the starting point of an action that continues up to now.

Some more examples:

• Moro aqui desde 2020. = I have lived here since 2020.
• Ela estuda desde cedo. = She has been studying since early morning.
• Estamos esperando desde ontem. = We have been waiting since yesterday.

So desde is a very common word when talking about duration starting at a point in time.

In Brazilian Portuguese, hoje de manhã is a very common, natural way to say this morning.

Literally, it is something like today in the morning.

Very common time expressions are:

• hoje de manhã = this morning
• hoje à tarde = this afternoon
• hoje à noite = tonight / this evening

You may also see:

• esta manhã

That exists, but hoje de manhã is often more common and conversational in Brazil.

So:

• desde hoje de manhã = since this morning

is very natural Brazilian Portuguese.

Portuguese often uses articles before nouns more often than English does, including before many technical terms, abbreviations, and letter-based expressions.

So:

• o pH = the pH

This sounds normal in Portuguese.

The masculine article o is commonly used with pH, and one reason often given is that it can be understood through the underlying noun potencial (potential), which is masculine:

• o potencial hidrogeniônico
• therefore o pH

Even if a learner does not know that technical expansion, the important practical point is simply:

• say o pH, not just pH, in sentences like this

In Brazilian Portuguese, pH is usually pronounced by naming the letters:

• pê agá

So the sentence would sound like:

• O aquarista monitora o pê agá desde hoje de manhã.

This is similar to how Portuguese often reads abbreviations letter by letter.

Yes. Portuguese allows some flexibility in word order, especially with time expressions.

Original:

• O aquarista monitora o pH desde hoje de manhã.

Also possible:

• Desde hoje de manhã, o aquarista monitora o pH.
• O aquarista, desde hoje de manhã, monitora o pH.

The original version is very natural and neutral.
Putting desde hoje de manhã at the beginning gives more emphasis to the time frame.

Not exactly.

• desde hoje de manhã focuses on a starting point and usually implies the action continues until now
• a partir de hoje de manhã means more literally starting this morning / from this morning onward

In your sentence, desde is the best choice because it matches the idea of an action that began earlier and is still going on.

Compare:

• Ele monitora o pH desde hoje de manhã.
  = He has been monitoring the pH since this morning.

• Ele vai monitorar o pH a partir de hoje de manhã.
  = He will monitor the pH starting this morning / from this morning onward.

So desde is the natural word for since here.

Yes. Both are possible, depending on context.

In isolation, monitora can mean:

• monitors
• is monitoring

But because the sentence includes desde hoje de manhã, English normally prefers:

• has been monitoring

So the Portuguese present tense covers an idea that English often expresses with a different tense.

That is a very common difference between the two languages.

Because of desde hoje de manhã, it is understood as an action that started this morning and continues now.

So this sentence is not mainly describing a general habit.

Without that time phrase, it could be habitual:

• O aquarista monitora o pH.
  = The aquarist monitors the pH. / The aquarist is monitoring the pH.

But once you add desde hoje de manhã, the ongoing meaning becomes much stronger:

• O aquarista monitora o pH desde hoje de manhã.
  = The aquarist has been monitoring the pH since this morning.

Yes, the sentence is grammatical and natural, especially in a technical or descriptive context.

A Brazilian might also say things like:

• O aquarista está monitorando o pH desde hoje de manhã.
• Ele está de olho no pH desde hoje de manhã.
  (He has been keeping an eye on the pH since this morning.)

Your original sentence sounds a bit more neutral or slightly formal because of monitora, but it is completely normal Portuguese.