Learn Portuguese (Portugal): Essa camisa está suja.

Portuguese has two ways to say 'that'. We use essa when the object is near the person you are talking to (for example, if they are holding or wearing the shirt). If the shirt was far away from both of you, you would use aquela instead.

Why do we use está instead of é for 'is' in this sentence?

Está comes from the verb estar, which we use for temporary states. A shirt being dirty is a temporary condition because you can wash it! If you used é (from the verb ser), it would sound like the shirt is permanently, fundamentally dirty.

Why does the word for dirty end in an 'a' (suja)?

In Portuguese, adjectives have to match the gender of the noun they describe. The word for shirt, camisa, is a feminine noun. Therefore, the default masculine adjective sujo changes to its feminine form, suja.

Does camisa refer to any kind of shirt, like a t-shirt?

Not quite! Camisa usually refers to a button-up shirt, like a dress shirt. If you wanted to say 'that t-shirt is dirty', you would use the English loanword and say Essa t-shirt está suja.