Subtraí dez.

It is the first-person singular preterite form of subtrair. In plain grammar terms, it is the I form of the simple past:

• eu subtraí = I subtracted

In European Portuguese, this tense is used for a completed action in the past.

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

Here, subtraí already tells you the subject is eu. So:

• Subtraí dez. = natural and normal
• Eu subtraí dez. = also correct, but more emphatic or contrastive

This is very common in Portuguese.

The accent is important because it changes both the stress and the grammar.

• subtraí = I subtracted
• subtrai = usually he/she subtracts or subtract! addressed to tu

So the accent helps distinguish the past eu form from other forms. It also shows that the stress falls on the last vowel: sub-tra-Í.

A good learner-friendly approximation is:

• subtraí → sub-tra-EE with the stress on the end
• dez → roughly desh at the end of a sentence in European Portuguese

So the whole sentence is approximately:

sub-tra-EE desh

A more precise note:

• the final z in dez is often pronounced like sh when it comes before a pause
• if another word followed and began with a vowel, that z would sound more like z

It follows a fairly predictable pattern, but it is not as straightforward as the most basic -ir verbs. It behaves like verbs such as trair and atrair.

Some useful forms are:

• eu subtraio = I subtract
• ele/ela subtrai = he/she subtracts
• eu subtraí = I subtracted
• ele/ela subtraiu = he/she subtracted

So subtraí is not random, but it belongs to a verb pattern that learners usually need to memorize.

It is correct and normal, but it sounds a bit technical or formal. You will often see or hear it in:

• maths
• accounting
• formal explanations
• written instructions

In casual conversation, people may choose a simpler verb depending on the context, but subtrair is the standard verb when you specifically mean to subtract.

It can definitely stand alone if the context is already clear.

Like English, Portuguese often leaves out information that both speakers already understand. So if you are already talking about a calculation, Subtraí dez. is a perfectly natural sentence.

If the context is not clear, you would usually add more information, for example by saying what total or amount you subtracted ten from.