Wolę trenować rano niż wieczorem.

Wolę is the 1st person singular form of the verb woleć.

So:

• woleć = to prefer
• wolę = I prefer

The ending -ę tells you the subject is I, which is why Polish does not need a separate word for I here.

After woleć, Polish normally uses the infinitive when the same person does both actions.

So:

• Wolę trenować = I prefer to train

This is the same pattern as in English: I prefer to train.

If the subject changed, Polish would usually use a clause instead, for example:

• Wolę, żebyś trenował rano. = I prefer that you train in the morning.

Because Polish often drops subject pronouns when they are already clear from the verb form.

• wolę already means I prefer

So Ja wolę trenować rano niż wieczorem is possible, but it sounds more emphatic, like:

• I prefer training in the morning, not someone else
• I prefer it, even if others don’t

Without ja, the sentence is more neutral.

Niż means than and is the normal word for comparisons like this one.

Here it compares two time expressions:

• rano = in the morning
• wieczorem = in the evening

So niż is the natural choice.

Od is more commonly used when comparing nouns, and it requires the genitive:

• Wolę kawę od herbaty. = I prefer coffee to tea.

In your sentence, where the comparison is between when you train, niż is the better choice.

Because rano is already an adverb meaning in the morning / in the early part of the day.

So Polish does not say w rano.
Just rano is correct.

This is very common with Polish time expressions. Also, Polish has no articles, so there is no separate word for the in the morning.

Wieczorem is the normal adverbial form meaning in the evening.

It comes from the noun wieczór (evening), but here Polish uses the form wieczorem to express when something happens.

So:

• wieczór = evening
• wieczorem = in the evening

You should learn wieczorem as a fixed, very common time expression.

Historically and grammatically, wieczorem is based on the instrumental singular form of wieczór.

But for a learner, the important point is simpler:

• wieczorem functions like an adverb
• it means in the evening

Polish often uses case forms this way in time expressions, for example:

• rankiem = in the morning
• latem = in summer
• zimą = in winter
• nocą = at night

So the -em is not caused by niż. It belongs to the expression wieczorem itself.

Because Polish time expressions are not all built the same way.

• rano is a fixed adverb
• wieczorem is a noun form used adverbially

Even though they look different, they both answer the same question:

• Kiedy? = When?

So they can be used together very naturally in one sentence.

No. Polish word order is fairly flexible.

Wolę trenować rano niż wieczorem is a neutral, natural version.

But you can move things around for emphasis:

• Rano wolę trenować niż wieczorem.
  This emphasizes rano.

• Trenować wolę rano niż wieczorem.
  This puts more focus on trenować or the contrast.

The original sentence is probably the best default version for most situations.

You could, and people would understand you, but wolę is the most natural and standard way to say I prefer.

Compare:

• Wolę trenować rano niż wieczorem. = the normal way to express preference
• Bardziej lubię trenować rano niż wieczorem. = more like I like training in the morning more than in the evening

So bardziej lubię is possible, but wolę is better here.

Not exactly.

• trenować often means to train, especially in a sports or fitness sense
• ćwiczyć can mean to exercise, to practice, or to do drills

So:

• trenować sounds very natural for regular physical training
• ćwiczyć is a bit broader

In many contexts both could work, but trenować is a good choice in this sentence.