Preciso de ajuda, pois a contabilidade é difícil.

Portuguese often drops subject pronouns when the verb form already makes the subject clear.
Preciso means I need, so eu is not necessary.

• Preciso de ajuda = I need help
• Eu preciso de ajuda = also correct, but more explicit or emphatic

In Brazilian Portuguese, using eu is common, but leaving it out is also very normal.

In standard Portuguese, precisar usually takes the preposition de when it is followed by a noun.

• preciso de ajuda = I need help
• preciso de dinheiro = I need money

But when precisar is followed by a verb in the infinitive, de is normally not used:

• preciso estudar = I need to study
• preciso sair = I need to leave

You may hear some Brazilians omit de in casual speech, but for learners, precisar de + noun is the safest and most standard pattern.

Ajuda means help or assistance. In this sentence, it is being used in the general, uncountable sense:

• Preciso de ajuda = I need help

You do not need an article here.
Preciso de uma ajuda can exist in some contexts, but it usually sounds more like I need a bit of help / a favor, and preciso de ajuda is the most natural general form.

Here, pois means because / since / as. It introduces the reason:

• Preciso de ajuda, pois a contabilidade é difícil.
• I need help, because/since accounting is difficult.

Compared with porque, pois often sounds a bit more formal, written, or explanatory. In everyday Brazilian speech, many people would more naturally say:

• Preciso de ajuda porque a contabilidade é difícil.

So yes, the meaning is very similar here, but pois has a slightly more formal tone.

In this sentence, pois introduces an explanation or reason for the first clause, so a comma is standard:

• Preciso de ajuda, pois a contabilidade é difícil.

This is especially common in more careful or written Portuguese.
With porque, people often do not use a comma:

• Preciso de ajuda porque a contabilidade é difícil.

So the comma helps show that the second part is an added explanation.

Portuguese often uses the definite article with general or abstract nouns more than English does.

So:

• a contabilidade = accounting
• literally, it looks like the accounting, but in English we usually just say accounting

This is very normal in Portuguese. The article does not always have to be translated into English.

Contabilidade usually means accounting as a field, subject, or area of work. Depending on context, it can refer to:

• the subject accounting
• the profession or practice of accounting
• an accounting department or accounting work

In this sentence, the most natural meaning is the subject or field: accounting is difficult.

Ser and estar can both be translated as to be, but they are used differently.

• é difícil suggests that accounting is difficult in a general or inherent sense
• está difícil suggests that it is difficult right now or under current circumstances

So:

• a contabilidade é difícil = accounting is difficult
• a contabilidade está difícil = accounting is difficult at the moment / seems hard right now

In your sentence, é difícil is the natural choice because it expresses a general opinion about accounting.

Some adjectives in Portuguese change form for gender, but others do not. Difícil is one of the adjectives that keeps the same form.

So you get:

• o trabalho é difícil
• a contabilidade é difícil

Both use difícil unchanged.

Only the article or noun shows the gender here, not the adjective itself.

The accent mark shows the stressed syllable and helps with pronunciation:

• difícil → stress on fí

So it is pronounced roughly like jee-FEE-seel in Brazilian Portuguese, though the exact sounds are different from English.

The accent is important in writing because it tells you where the stress goes and follows Portuguese spelling rules.

It would be understood, but it is less natural in standard Portuguese because contabilidade normally takes the article here.

The more natural version is:

• Eu preciso de ajuda porque a contabilidade é difícil.

Also, porque is more common than pois in everyday conversation, so this version sounds very natural in Brazilian Portuguese.

Yes, it is grammatical and natural. It sounds a little more neutral or slightly formal because of pois.

A very common everyday Brazilian version would be:

• Preciso de ajuda porque a contabilidade é difícil.

Your original sentence is still perfectly good Portuguese; it just has a slightly more written or formal feel.