Eu falei com o detetive.

A word-by-word breakdown:

• Eu = I
• falei = spoke / talked
• com = with
• o = the (masculine singular)
• detetive = detective

So the structure is literally:

I spoke with the detective.

Falei is the 1st person singular form of the verb falar in the pretérito perfeito, which is usually called the simple past or preterite in English explanations.

So:

• eu falo = I speak / I am speaking
• eu falei = I spoke / I talked

This tense usually describes a completed action in the past.

You can absolutely say Falei com o detetive.

Portuguese often drops subject pronouns because the verb ending already shows who the subject is. In falei, the ending -ei already tells us it is I.

So both are correct:

• Eu falei com o detetive.
• Falei com o detetive.

Including eu can add:

• emphasis
• contrast
• clarity, if needed

For example:

• Eu falei com o detetive, não ela.
  I talked to the detective, not her.

In Portuguese, it is very common to use a definite article like o or a where English might not focus on it as much.

com o detetive means with the detective, referring to a specific detective.

If you said com um detetive, that would mean with a detective.

Saying just com detetive sounds unnatural in normal Portuguese in this sentence.

No, in standard written Portuguese, com o stays as two words.

So you write:

• com o detetive

not a contraction.

However, Portuguese does have special forms of com with some pronouns:

• comigo = with me
• contigo = with you
• consigo = with him/her/you
• conosco = with us

But with articles like o, there is no standard written contraction.

Detetive can refer to either a man or a woman, but the article shows the gender of the person being referred to.

• o detetive = the male detective
• a detetive = the female detective

The noun detetive itself usually does not change form.

So:

• Eu falei com o detetive. = I spoke with the male detective.
• Eu falei com a detetive. = I spoke with the female detective.

Because falar com alguém means to speak/talk with someone.

This is a very common pattern:

• falar com alguém = talk with someone
• falar para alguém = talk to someone
• falar a alguém = speak to someone

In everyday Brazilian Portuguese, falar com and falar para are both very common, but falar com is especially natural for the idea of having a conversation or speaking with someone.

Not exactly.

• falei comes from falar = to speak / to talk
• disse comes from dizer = to say / to tell

Compare:

• Eu falei com o detetive. = I spoke with the detective.
• Eu disse isso ao detetive. = I said that to the detective.

So falei com focuses on the act of speaking or talking, while disse usually focuses more on what was said.

In Brazilian Portuguese, falei is roughly pronounced fah-LEI, with the stress on the last part.

A rough guide:

• fa = like fa in father, but shorter
• lei = sounds somewhat like lay

So:

• falei ≈ fah-LEI

The whole sentence is roughly:

• Eu falei com o detetive
• approximately eh-ooh fah-LEI kohng oo deh-teh-CHEE-vee

Pronunciation varies by region, but that rough version is good enough for a learner.

Yes, but the most natural neutral order is:

• Eu falei com o detetive.

You can also say:

• Falei com o detetive.

Other orders are possible, but they usually sound more marked or emphasize something:

• Com o detetive, eu falei.

That sounds more like As for the detective, I spoke with him or gives special emphasis.

So for normal everyday use, stick with:

• Eu falei com o detetive.
• Falei com o detetive.

Yes, depending on the nuance.

For example:

• Eu conversei com o detetive. = I had a conversation with the detective.
• Eu falei com o detetive. = I spoke with the detective.
• Eu disse ao detetive... = I told the detective...
• Eu perguntei ao detetive... = I asked the detective...

So falei is a very natural, general way to express that you spoke to or with the detective.