Kad imam prehladu, čelo mi je toplo i često počnem kihati već ujutro.

Both are possible, but they mean slightly different things.

• Imam prehladu = I have a cold
• Prehlađen / prehlađena sam = I am congested / I have caught a cold / I’m suffering from a cold

In this sentence, imam prehladu is a very natural way to say when I have a cold. It focuses on the illness as something you have.

Also note:

• prehladu is the accusative singular form of prehlada, because it is the direct object of imam.

Because imati (to have) takes a direct object, and direct objects usually go in the accusative case.

So:

• nominative: prehlada = a cold
• accusative: prehladu = a cold as the object of the verb

Examples:

• Imam prehladu. = I have a cold.
• Prehlada je neugodna. = A cold is unpleasant.

Yes. Kad can mean both:

• when in a statement/subordinate clause
  
  • Kad imam prehladu... = When I have a cold...
• when? in a question
  
  • Kad dolaziš? = When are you coming?

In this sentence, kad introduces a time clause: when I have a cold.

You may also see kada, which means the same thing but sounds a bit more formal or fuller:

• Kad imam prehladu...
• Kada imam prehladu...

Both are correct.

Croatian very often uses a short dative pronoun to show possession, especially with body parts.

So:

• čelo mi je toplo literally = the forehead is warm to me
• natural English meaning = my forehead is warm

This is often more natural in Croatian than using the possessive adjective moje.

Compare:

• Boli me glava. = My head hurts.
• Ruke su mi hladne. = My hands are cold.
• Čelo mi je toplo. = My forehead is warm.

You can say moje čelo je toplo, but it is usually less natural unless you want emphasis, such as my forehead, not someone else’s.

Mi is the short dative form of ja (I / me).

Here it marks the person affected or the possessor:

• mi = to me / my

So:

• čelo mi je toplo literally = the forehead is warm to me
• idiomatic English = my forehead is warm

This structure is extremely common with:

• body parts
• physical states
• personal belongings in context

Examples:

• Oči su mi umorne. = My eyes are tired.
• Noga me boli. = My leg hurts.
• Glas mi je promukao. = My voice is hoarse.

Because čelo is a neuter noun, and the adjective has to agree with it.

• čelo = neuter singular
• adjective must also be neuter singular: toplo

Compare the adjective topao / topla / toplo:

• masculine: topao
• feminine: topla
• neuter: toplo

Examples:

• Nos je topao. = The nose is warm.
• Ruka je topla. = The hand is warm.
• Čelo je toplo. = The forehead is warm.

Both are possible, but they focus on different things.

• često kiham = I often sneeze
  This describes a repeated action in general.
• često počnem kihati = I often start sneezing
  This focuses on the beginning of the sneezing.

So počnem kihati adds the idea that sneezing starts, especially at a particular time.

In this sentence, that works well with već ujutro:

• često počnem kihati već ujutro = I often start sneezing already/as early as in the morning

After početi (to begin/start), Croatian commonly uses an infinitive.

So:

• počnem kihati = I start to sneeze / I start sneezing

Structure:

• počnem = I begin/start
• kihati = to sneeze

Other examples:

• Počinjem učiti. = I’m starting to study.
• Počeo je padati snijeg. = It started snowing.
• Počela je plakati. = She started crying.

Yes, but in this sentence it is closer to already, as early as, or even depending on context.

• već ujutro suggests that the sneezing starts as early as the morning
• It adds a nuance of earlier than expected or surprisingly early

So:

• počnem kihati već ujutro = I start sneezing already in the morning
• more natural English: I start sneezing as early as the morning or I start sneezing already in the morning

Without već, the sentence would simply say the sneezing starts in the morning, without that extra nuance.

In standard usage here, ujutro is the normal adverb meaning in the morning.

• ujutro = in the morning

Examples:

• Dolazim ujutro. = I’m coming in the morning.
• Već ujutro kašljem. = I’m already coughing in the morning.

The separated form u jutro is generally not what you want in this everyday time expression. Learners should normally remember ujutro as a fixed adverb.

Croatian word order is flexible, but not completely free.

The version in the sentence,

• čelo mi je toplo

is very natural.

A few things matter here:

1. Short pronouns like mi tend to appear in the second position area of the clause.
2. The verb je is also a clitic and follows similar placement rules.
3. Croatian often places these short unstressed words early in the clause.

So:

• Čelo mi je toplo. = natural
• Mi je čelo toplo. = possible only in special contexts, but not the neutral choice
• Čelo je mi toplo. = not correct standard clitic placement

A good learner rule is:

• keep short pronouns like mi, ti, mu, joj, se near the beginning of the clause, in the usual clitic position.

Because Croatian usually does not need subject pronouns when the verb form already shows the subject.

• imam = I have
• počnem = I begin/start

The endings tell you it is I, so ja is normally omitted.

Compare:

• Imam prehladu. = I have a cold.
• Počnem kihati. = I start sneezing.

You would add ja only for emphasis or contrast:

• Ja imam prehladu, a on nema. = I have a cold, but he doesn’t.

It sounds like a general repeated situation, not one specific occasion.

That meaning comes from several things:

• Kad imam prehladu = Whenever/When I have a cold
• često = often
• present tense in Croatian can describe repeated or habitual actions

So the sentence means something like:

• Whenever I have a cold, my forehead gets warm and I often start sneezing already in the morning.

If you wanted one specific past situation, Croatian would usually use past tense forms instead.

Kihati is the normal imperfective verb meaning to sneeze in a general or ongoing sense.

It can refer to:

• sneezing in general
• repeated sneezing
• the action of sneezing as a process

So in počnem kihati, it means:

• I start sneezing not just
• I sneeze once

That makes sense because colds usually cause repeated sneezing, not a single isolated sneeze.