Mein T‑Shirt ist heute sauber.

Because T‑Shirt is grammatically neuter in German (das T‑Shirt).

In the nominative singular:

• masculine: mein Pullover (der Pullover)
• neuter: mein T‑Shirt (das T‑Shirt)
• feminine: meine Hose (die Hose)

So you use mein (not meine) with neuter and masculine nouns in the nominative singular.

With many loanwords like T‑Shirt, you usually just have to learn the gender. Dictionaries always list nouns with their article:

• das T‑Shirt – neuter
• der Pullover – masculine
• die Bluse – feminine

Over time you’ll get a feel for common patterns, but for specific items of clothing you generally memorize the article together with the noun: das T‑Shirt.

In German:

• All nouns are capitalized: T‑Shirt
• Adjectives are normally lower‑case: sauber
• Adverbs and other small words are also lower‑case: heute

So the capitalization here simply follows the general rule: nouns big, everything else small (with a few special exceptions you’ll see later).

Sauber is used here as a predicate adjective after the verb sein (to be):

• Mein T‑Shirt ist sauber. – My T‑shirt is clean.

Predicate adjectives in German do not take endings. They stay in the base form:

• Die Hose ist neu. (neu, not neue)
• Die Schuhe sind teuer. (teuer, not teuren)

Adjective endings appear when the adjective comes before a noun:

• ein sauberes T‑Shirt (a clean T‑shirt)
• mein sauberes T‑Shirt

In normal German main clauses, the finite verb must be in second position:

• Mein T‑Shirt ist heute sauber. ✔️

Here the “slots” are:

1. Mein T‑Shirt (first element)
2. ist (verb in 2nd position)
3. heute sauber (rest of the sentence)

Putting heute between the subject and the verb like Mein T‑Shirt heute ist sauber sounds wrong/orderly broken in standard German. The verb must directly follow the first element.

Yes, adverbs of time such as heute are quite flexible. These are all possible, with slightly different emphasis:

• Mein T‑Shirt ist heute sauber. (neutral; very common)
• Heute ist mein T‑Shirt sauber. (emphasizes today)
• Mein T‑Shirt ist sauber heute. (understandable, but less natural in standard German; native speakers would usually prefer the first two)

The key rule you must keep: the verb stays in 2nd position. So if you start with Heute, the verb comes right after it:

• Heute ist mein T‑Shirt sauber. ✔️
• Heute mein T‑Shirt ist sauber. ❌

Heute is a stand‑alone adverb meaning today. You don’t add a preposition:

• heute – today
• morgen – tomorrow
• gestern – yesterday

You use am with more specific day words:

• am Montag – on Monday
• am 1. Januar – on January 1st

So:

• Mein T‑Shirt ist heute sauber. ✔️
• Mein T‑Shirt ist am Montag sauber. ✔️
• Mein T‑Shirt ist am heute sauber. ❌

The subject of a sentence is in the nominative case in German.
In Mein T‑Shirt ist heute sauber, the T‑shirt is the thing that “is clean,” so it’s the subject → nominative.

With the verb sein (to be), both sides of the ist are in nominative:

• Mein T‑Shirt (nominative) ist sauber (adjective, no case)

If it were a different verb requiring an object, then you might see other cases (accusative, dative, etc.), but not here.

Both sauber and rein can mean clean, but they’re used a bit differently:

• sauber – not dirty; washed; tidy enough for normal use

  • Mein T‑Shirt ist heute sauber. (sounds completely normal)

• rein – pure / completely clean / often more technical or poetic

  • reine Luft – pure air
  • reines Gold – pure gold

You could say Mein T‑Shirt ist rein, but in everyday speech sauber is the normal word for clothes being clean. Rein would sound a bit unusual here.

German has simply borrowed the English word T‑shirt and adapted its spelling conventions a bit:

• It keeps the T‑
  • ‑Shirt structure.
• As a noun, it must be capitalized: T‑Shirt.
• It takes a German article and grammar: das T‑Shirt, die T‑Shirts.

You will see some spelling variation in informal writing, but T‑Shirt with a capital T and a hyphen is the standard dictionary form.

You need plural for both the noun and the verb, and the possessive changes too:

• Meine T‑Shirts sind heute sauber.

Changes compared to the singular:

• Mein → Meine (because T‑Shirts is now plural)
• T‑Shirt → T‑Shirts
• ist → sind (3rd person plural of sein)

So:

• Singular: Mein T‑Shirt ist heute sauber.
• Plural: Meine T‑Shirts sind heute sauber.