Перед ужином я начинаю размораживать рыбу.

Because перед normally requires the instrumental case.

So:

• ужин = nominative dictionary form
• ужином = instrumental singular

In time expressions, перед + instrumental means before something:

• перед ужином = before dinner
• перед работой = before work
• перед сном = before sleep / before going to bed

So перед ужином is the normal grammatical form here.

It can mean both, depending on context.

• перед домом = in front of the house
• перед ужином = before dinner

In this sentence, because ужин is an event/time, the meaning is clearly before dinner, not a physical location.

Because рыба is the direct object of размораживать.

The verb размораживать means to defrost / thaw, and the thing being defrosted goes in the accusative case.

• рыба = nominative
• рыбу = accusative singular

Since рыба is feminine, the accusative singular changes -а to -у.

Compare:

• Я ем рыбу. = I eat fish.
• Я покупаю рыбу. = I buy fish.
• Я размораживаю рыбу. = I defrost fish.

Because after начинать / начать, Russian normally uses the imperfective infinitive.

So:

• начинаю размораживать = I start defrosting
• not normally начинаю разморозить

This is because начинать focuses on the beginning of a process, and the infinitive after it is usually something ongoing or repeatable, which is what the imperfective expresses.

Here:

• размораживать = imperfective, process of defrosting
• разморозить = perfective, to defrost completely / finish defrosting

So начинаю размораживать sounds natural: I begin the process of defrosting the fish.

They are an aspect pair:

• размораживать = imperfective
• разморозить = perfective

Very roughly:

• размораживать focuses on the process, repetition, or ongoing action
• разморозить focuses on completion or a single finished result

Examples:

• Я размораживаю рыбу. = I am defrosting fish / I defrost fish.
• Я разморозил рыбу. = I defrosted the fish completely.

In your sentence, the action is just starting, so the imperfective размораживать is the expected choice.

Yes, Russian often drops subject pronouns when they are clear from the verb ending.

So both are possible:

• Перед ужином я начинаю размораживать рыбу.
• Перед ужином начинаю размораживать рыбу.

The verb начинаю already shows I.

Including я can make the subject clearer or slightly more emphatic. In many contexts, leaving it out would sound perfectly natural.

Начинаю is the 1st person singular present tense of начинать.

So:

• начинаю = I begin / I start

In this sentence it means:

• I start defrosting the fish
• or, depending on context, I usually start defrosting fish before dinner

Russian present-tense forms can sometimes describe a habitual action as well as something happening now.

It could be either, depending on context.

Russian present tense can express:

1. A current action

   • Before dinner, I’m starting to defrost the fish.

2. A habitual/repeated action

   • Before dinner, I start defrosting fish.

Without extra context, both readings are possible. English usually forces you to choose more clearly than Russian does.

Yes. Russian word order is flexible, because case endings show the grammatical roles.

Your sentence:

• Перед ужином я начинаю размораживать рыбу.

Other possible orders:

• Я перед ужином начинаю размораживать рыбу.
• Рыбу я начинаю размораживать перед ужином.

These all mean roughly the same thing, but the emphasis changes:

• Перед ужином... puts time first
• Я... puts the subject first
• Рыбу... highlights the fish

The original sentence is very natural if you want to frame the action by time: Before dinner...

Russian does not have articles, so рыбу can mean:

• a fish
• the fish
• fish

The exact meaning depends on context.

So this sentence could be understood as:

• Before dinner, I start defrosting the fish
• Before dinner, I start defrosting fish

If the speaker and listener already know which fish is meant, English would often use the fish.

Ужин usually refers to the evening meal.

Depending on the variety of English and context, it may be translated as:

• dinner
• supper

In most learner translations, dinner is the default choice, but supper may fit better in some households or dialects.

By itself, перед ужином just means before dinner. It does not specify exactly how long before.

It could mean:

• a little before dinner
• sometime earlier, leading up to dinner

If you want to be more specific, Russian can add words such as:

• прямо перед ужином = right before dinner
• незадолго до ужина = shortly before dinner

So the original phrase is neutral and general.