Eu costumo consultar o dossiê da turma quando não encontro o separador certo.

Costumo is the 1st person singular of costumar.

In European Portuguese, costumar + infinitive means to usually / tend to / be in the habit of doing something.

So:

• Eu costumo consultar... = I usually consult / I tend to consult...

It is a very common structure for talking about habits:

• Costumo estudar à noite. = I usually study at night.
• Ela costuma chegar cedo. = She usually arrives early.

It is often more natural in Portuguese than translating usually word-for-word.

No, eu is optional here.

Portuguese often leaves out subject pronouns because the verb ending already shows who the subject is:

• Eu costumo consultar...
• Costumo consultar...

Both mean I usually consult...

You include eu mainly for:

• emphasis
• contrast
• clarity, if needed

For example:

• Eu costumo consultar o dossiê, mas ele não.
  = I usually consult the file, but he doesn’t.

So in this sentence, eu is grammatically correct but not required.

It is similar, but in everyday Portuguese consultar often means:

• to look something up
• to refer to
• to check
• to consult

In this sentence, consultar o dossiê suggests looking at the file or class record to get information.

So it is not necessarily as formal as English consult can sound. In context, it may be closer to:

• I check the class file
• I refer to the class dossier
• I look in the class file

Dossiê usually means a file, set of documents, or dossier.

In a school or office context, it can refer to a collection of papers or records kept together. So o dossiê da turma is likely:

• the class file
• the class dossier
• the file for that class/group

It is a masculine noun, which is why it takes o:

• o dossiê

Because de + a contracts to da in Portuguese.

So:

• de + a turma → da turma

This is extremely common and mandatory in normal Portuguese.

Other common contractions are:

• de + o → do
• em + a → na
• em + o → no
• a + a → à

So o dossiê da turma literally means the dossier/file of the class.

Turma usually means a class, group, or set of students.

In school contexts in Portugal, turma is commonly used for a class group:

• a turma do 7.º ano = the Year 7 class
• a minha turma = my class

It is a feminine noun, which is why the article inside da is a:

• da turma = of the class

In European Portuguese, separador often means a divider, tab, or sometimes a section marker in a folder or binder.

In this sentence, o separador certo likely means:

• the right divider
• the correct tab
• the right section

This is a word that may feel a little different from what an English speaker expects, because the exact translation depends on the physical object and context.

It is also a masculine noun:

• o separador

Because Portuguese uses the present tense for habitual actions, just like English often does.

• quando não encontro o separador certo
  = when I don’t find the right divider/tab

This does not have to mean only right now. It can describe something that happens repeatedly.

The whole sentence expresses a habit:

• Eu costumo consultar... quando não encontro...
  = I usually consult... when I don’t find...

So both verbs in the present tense are perfectly natural.

In Portuguese, não normally goes before the conjugated verb to make it negative.

So:

• encontro = I find
• não encontro = I do not find / I can’t find

This is the standard position:

• Não sei. = I don’t know.
• Não quero ir. = I don’t want to go.
• Quando não encontro... = When I don’t find...

In Portuguese, adjectives often come after the noun.

So:

• o separador certo = the right/correct divider

Also, with certo, position can change the meaning:

• o separador certo = the correct/right divider
• um certo separador = a certain divider

So in your sentence, certo after the noun clearly means right/correct.

Portuguese uses definite articles more often than English.

Here, the speaker is referring to specific things:

• o dossiê da turma = the class file
• o separador certo = the right divider/tab

This sounds natural in Portuguese. English sometimes uses fewer articles, but Portuguese often prefers them when referring to identifiable objects.

Yes. Portuguese word order is fairly flexible.

You could also say:

• Quando não encontro o separador certo, costumo consultar o dossiê da turma.

This means the same thing. The difference is mainly one of focus or style:

• starting with Eu costumo... emphasizes the habit first
• starting with Quando não encontro... emphasizes the situation first

Both are natural.

A rough guide:

• dossiê ≈ do-see-AY
• não ≈ a nasal sound, something like nowng but without a full final g
• encontro ≈ en-KON-tru in European Portuguese, with reduced final vowel

A few useful pronunciation points:

• ê in dossiê marks a stressed vowel.
• ão in não is nasal, which is often difficult for English speakers at first.
• In European Portuguese, unstressed final -o often sounds closer to u than to English oh.

So encontro often sounds closer to encontru than an English speaker might expect.