Está frio, por isso eu preciso de um casaco.

It is cold, so I need a coat.

Breakdown of Está frio, por isso eu preciso de um casaco.

• eu: I
• estar: to be
• um: a
• frio: cold
• precisar de: to need
• o casaco: the coat
• por isso: so

Questions & Answers about Está frio, por isso eu preciso de um casaco.

Why do we say está frio instead of faz frio?

In Brazilian Portuguese both are possible, but they carry a slight nuance:

• Está frio uses the verb estar (to be) to describe the current state of the environment: “It is cold right now.”
• Faz frio uses the verb fazer (to make/do) in an impersonal way: “It’s cold” as a general weather statement. Native speakers often prefer está frio when focusing on how you feel at the moment, and faz frio when reporting the weather in general.

What’s the difference between saying está frio and estou com frio?

• Está frio literally means “it’s cold,” referring to the weather or surroundings.
• Estou com frio literally means “I am with cold,” i.e. “I feel cold.”
  Use estou com frio when you talk about your own sensation (“I’m cold”), and está frio to describe the temperature objectively.

What does por isso mean, and how is it used here?

Por isso means “for that reason” or “therefore.” It connects a cause (it’s cold) to its effect (you need a coat). In the sentence:  Está frio, por isso eu preciso de um casaco.
you’re literally saying: “It’s cold, therefore I need a coat.”

Can I replace por isso with então, logo, or portanto?

Yes, all of these are connectors of consequence, but with different register:

• então – more colloquial (“…então eu preciso…”)
• logo – neutral/formal (“…logo eu preciso…”)
• portanto – formal/written (“…portanto eu preciso…”)
  Each one emphasizes the result in slightly different styles, but the meaning stays the same.

Why is there a comma before por isso?

The comma sets off an adverbial connector. Whenever you join two independent clauses with a linking phrase like por isso, you typically insert a comma:  Clause 1, por isso clause 2.
It helps the sentence flow and shows the logical break between cause and effect.

Why do we say preciso de um casaco instead of preciso um casaco?

In Portuguese the verb precisar (to need) requires the preposition de before its object:  precisar de algo/alguém.
So you always say precisar de + noun. Omitting de would be ungrammatical.

Why is the pronoun eu included here? Can I omit it?

Portuguese is a pro-drop language: the subject pronoun is often dropped because the verb ending makes it clear who you are.

• With pronoun: eu preciso de um casaco (adds emphasis or clarity).
• Without pronoun: preciso de um casaco (more natural in speech).
  Both are correct; include eu when you want to stress “I.”

Why is there an indefinite article um before casaco? Can I say preciso de casaco?

Using um casaco (“a coat”) is standard when you need one unspecified item.
You could drop the article in some set expressions (e.g., preciso de roupa – “I need clothing”), but with countable items like casaco you normally keep the article:  preciso de um casaco = “I need a (one) coat.”
Omitting it (preciso de casaco) sounds incomplete or poetic, not everyday Portuguese.