Η εξέταση της ένστασης ελέγχει την πράξη βεβαίωσης παράβασης.

The review of the objection checks the formal violation notice.

Breakdown of Η εξέταση της ένστασης ελέγχει την πράξη βεβαίωσης παράβασης.

• ελέγχω: to check
• η πράξη βεβαίωσης παράβασης: the formal notice recording the violation
• η εξέταση της ένστασης: the review of the objection

Questions & Answers about Η εξέταση της ένστασης ελέγχει την πράξη βεβαίωσης παράβασης.

What is the subject of this sentence?

The subject is Η εξέταση της ένστασης.

The head noun is εξέταση and it is in the nominative singular, so that is what the verb agrees with.
The part της ένστασης is attached to it and means of the objection / appeal, but it is not the grammatical subject by itself.

So the structure is:

• Η εξέταση της ένστασης = the review/examination of the objection
• ελέγχει = checks/reviews
• την πράξη βεβαίωσης παράβασης = the act/report of certification of a violation

Why are there three different forms of the word the: η, της, and την?

Because Greek articles change depending on case, gender, and number.

Here they are all feminine singular, but in different cases:

• η = nominative feminine singular
  used for the subject
  → η εξέταση

• της = genitive feminine singular
  often used for of
  → της ένστασης

• την = accusative feminine singular
  used for the direct object
  → την πράξη

So English uses one word, the, but Greek changes the form.