Ich bestelle das T-Shirt für morgen vor.

Because the full verb is vorbestellen, which is a separable-prefix verb.

In a normal main clause:

• the conjugated part goes in second position: bestelle
• the prefix goes to the end: vor

So:

• Ich bestelle das T-Shirt für morgen vor.

But in an infinitive or in some subordinate clauses, the verb stays together:

• Ich möchte das T-Shirt für morgen vorbestellen.
• ..., weil ich das T-Shirt für morgen vorbestelle.

The full verb is vorbestellen.

Its parts are:

• vor- = prefix
• bestellen = main verb part

In the sentence, it is conjugated for ich in the present tense:

• ich bestelle ... vor

So the pattern is:

• ich bestelle vor
• du bestellst vor
• er/sie/es bestellt vor

German main clauses usually follow the verb-second rule (V2). That means the finite, conjugated verb must come in the second position of the clause.

Here:

• Ich = first element
• bestelle = second element

Then the rest follows:

• das T-Shirt
• für morgen
• vor

So even though English and German can both start with I / Ich, the important grammar point is that the conjugated verb must be second.

T-Shirt is a neuter noun in German, so its definite article is das.

Here it is the direct object of the verb, and in the singular, the accusative form of das is still das.

So:

• nominative: das T-Shirt
• accusative: das T-Shirt

That is why you do not see any change in the article here.

It is in the accusative case because it is the thing being ordered — the direct object.

A useful question is:

• What am I pre-ordering?
• das T-Shirt

That makes das T-Shirt the accusative object of vorbestellen.

In this sentence, the article looks the same as nominative because the noun is neuter:

• nominative: das T-Shirt
• accusative: das T-Shirt

Für morgen is a time expression. It tells you for when the pre-order is intended.

It is not the main object of the verb; it adds extra information about the timing.

A learner-friendly way to think about it is:

• das T-Shirt = what is being pre-ordered
• für morgen = for what time/day

German often uses short time expressions like this very naturally.

Because German very often uses the present tense to talk about the future when the time is already clear.

Here, the future meaning is obvious because of für morgen.

So German commonly says:

• Ich bestelle das T-Shirt für morgen vor.

Instead of needing a special future form every time.

You could also form the future with werden, but it is often unnecessary:

• Ich werde das T-Shirt für morgen vorbestellen.

That is grammatical, but the simple present is usually more natural in everyday German when a future time expression is already present.

Yes.

• bestellen = to order
• vorbestellen = to pre-order / to order in advance

So vorbestellen adds the idea that the order is being placed ahead of time.

That prefix vor- often carries the sense of before or in advance.

Yes. German word order is flexible, but the conjugated verb still has to stay in second position in a main clause, and the separable prefix still goes to the end.

For example, these are also possible:

• Für morgen bestelle ich das T-Shirt vor.
• Das T-Shirt bestelle ich für morgen vor.

These versions shift the emphasis:

• starting with Für morgen highlights the time
• starting with Das T-Shirt highlights the object

The original sentence is a neutral, very natural word order.

In a subordinate clause, the verb no longer follows the normal verb-second pattern. Instead, the full verb usually comes at the end, and the separable verb is written together.

For example:

• ..., weil ich das T-Shirt für morgen vorbestelle.

Notice the difference:

Main clause:

• Ich bestelle das T-Shirt für morgen vor.

Subordinate clause:

• ..., weil ich das T-Shirt für morgen vorbestelle.

So the separation disappears there.

Because all German nouns are capitalized, and T-Shirt is a noun.

So even if a word comes from English, once it is used as a noun in German, it is capitalized:

• das T-Shirt

The hyphen is also part of the normal spelling.

Yes, but it does not mean exactly the same thing.

• für morgen = for tomorrow, meaning the order is intended for tomorrow
• morgen = tomorrow, meaning the action takes place tomorrow

So compare:

• Ich bestelle das T-Shirt für morgen vor.
  The pre-order is for tomorrow.

• Ich bestelle das T-Shirt morgen vor.
  I will pre-order the T-shirt tomorrow.

That is an important difference.