Gisteraand was die vuur groot, omdat die hout baie droog was.

Afrikaans often puts a time word or phrase first, just like English can do with Last night.

So:

• Gisteraand was die vuur groot
• literally: Last night was the fire big

But in natural English we would say Last night the fire was big/large.

This also shows an important Afrikaans word-order rule: when something other than the subject comes first, the finite verb usually comes second. That is why was comes right after Gisteraand.

Because Afrikaans follows the verb-second rule in main clauses.

That means the conjugated verb usually stays in the second position:

• Die vuur was groot.
• Gisteraand was die vuur groot.

When die vuur is first, was comes second.
When Gisteraand is first, was still comes second.

This is very common in Afrikaans and is one of the first word-order patterns English speakers need to get used to.

Gisteraand means last night or yesterday evening, depending on context.

It is made from:

• gister = yesterday
• aand = evening

As one word, gisteraand is the normal Afrikaans form.

Both clauses are talking about a past situation, and was is the past tense of wees (to be).

So:

• die vuur groot was = the fire was big/large
• die hout baie droog was = the wood was very dry

Afrikaans uses was for the past of to be, much like English was/were.

This is a bit of a special case.

Afrikaans often uses a perfect-like past with het + ge- for many verbs:

• Ek het gewerk = I worked / I have worked

But the verb wees (to be) commonly has its own simple past form:

• is = is
• was = was

So in this sentence, was is completely normal.

Yes, groot literally means big or large.

In this sentence, die vuur was groot most naturally means something like:

• the fire was big
• the fire was large
• possibly the fire was strong/blazing, depending on context

So it is literal, but in English we might choose a slightly more natural wording depending on the situation.

Here, die simply means the.

Afrikaans uses die as the definite article for nouns in general. Unlike some other languages, Afrikaans does not change the for gender the way Dutch or German does.

So:

• die vuur = the fire
• die hout = the wood

This makes articles in Afrikaans relatively simple for English speakers.

Because droog comes after the noun as part of the predicate, not directly before the noun.

Compare:

• die hout was baie droog = the wood was very dry
  
  • here droog is a predicate adjective, so it stays droog

But:

• die baie droë hout = the very dry wood
  
  • here the adjective comes before the noun, so it often takes -e

This is a very useful pattern in Afrikaans:

• after is/was → usually base form: groot, droog, mooi
• before a noun → often -e form: groote? actually usually groot stays groot in standard Afrikaans, but many adjectives do add -e, such as droë

A clear pair is:

• Die hout is droog
• die droë hout

Omdat means because.

It introduces a reason:

• Gisteraand was die vuur groot = Last night the fire was big
• omdat die hout baie droog was = because the wood was very dry

So the second part explains the cause of the first part.

Because omdat introduces a subordinate clause, and subordinate clauses in Afrikaans usually send the finite verb toward the end.

Main clause:

• Die hout was baie droog.

After omdat:

• omdat die hout baie droog was

Notice how was moves to the end of that clause.

This is a very important pattern:

• Ek bly by die huis, want dit reën.
• Ek bly by die huis, omdat dit reën.

With omdat, the verb is typically final in the subordinate clause.

Yes, often you can, but the structure changes.

With want:

• Gisteraand was die vuur groot, want die hout was baie droog.

With omdat:

• Gisteraand was die vuur groot, omdat die hout baie droog was.

The difference is mainly grammatical here:

• want behaves more like a coordinating conjunction, so normal main-clause word order stays
• omdat introduces a subordinate clause, so the verb moves to the end

Both can mean because, but omdat often sounds a bit more explicitly causal or formal.

Here hout means wood as a material, so it works like a mass noun, just like in English.

So:

• die hout = the wood

It does not mean the woods or necessarily pieces of wood one by one. It refers to the wood as fuel/material.

That is why the sentence means the fire was big because the wood was very dry.

Yes, it is natural and grammatical.

A native speaker might also say similar things like:

• Die vuur was gisteraand groot, omdat die hout baie droog was.
• Gisteraand was die vuur groot, want die hout was baie droog.

Your original sentence is perfectly good Afrikaans and is especially useful because it shows two important grammar patterns at once:

• verb second in the main clause
• verb at the end after omdat