The easy way to ielts writing academic Module

No comma is required if the complex sentence ends with the
dependent clause.
In all the sentences above, the clauses starting with a subordinating
conjunction or a relative pronoun are dependent (underlined sections)
because their meanings are not complete on their own, and they rely on the
independent clauses (sections that are not underlined) to be completely
meaningful.
4) Compound-complex sentences
Compound-complex sentences are simply a combination of one compound
sentence and one complex sentence. These structures produce long sentences,
which are quite valuable in academic writing as they provide better fluency
and flow of ideas; however, keeping a balance in using them is vital as
overusing compound-complex sentences could create confusion in writing.
For example
She finished her assignment, but she did not have time to review her
lessons 
even though she had a test the day after.
He likes to tidy up his room, but he does not like to organize his
closet 
because he finds it time-consuming.
I saw her again at work, and I recognized her from the other night
when we were at the party.
There is one compound sentence (in black) connected to a dependent clause

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(in blue) by a subordinating conjunction or a relative pronoun forming a
compound-complex structure in all these examples.
Prepositions
A common area of grammatical mistakes in the IELTS writing test is around
“prepositions.” Some prepositions, also known as 
dependent prepositions
,
collocate nouns, verbs, or adjectives, and some of them were briefly
discussed in the “collocations” section. Other prepositions are classified as
independent prepositions
under different categories. The most common
independent prepositions 
are as follows: