It mostly exists in grammatically feminine words. It always follows a fatḥaـَ ("a"), and it normally has no other sound in modern Arabic. However, a /t/ is added in genitive construct (إِضَافَة(ʾiḍāfa)), and it changes to a normal ت(tāʾ) when a suffix is added, e.g. غُرْفَة(ḡurfa, “a room”) -> غُرْفَتِي(ḡurfatī, “my room”) (غُرْفَة(ḡurfa) (ة -> ت) + ـِي(-ī)).
When desinential inflection is used in Classical Arabic or in formal Modern Standard Arabic, non-pausal ة is pronounced as /t/ (preceded by usually unwritten fatḥa "a") and has -u/un, -i/-in, -a/an case endings, e.g. لُغَةٌ - "luḡa" (pausal and informal), "luḡatun" formal, non-pausal pronunciation (nominative case, indefinite).
Adverbials from tāʾ marbūṭa do not add alif (ا) with fatḥatān. Instead, fatḥatān is attached directly to tāʾ marbūṭa and the "hidden t" is pronounced with the ending "-an" e .g. عَادَةً(ʿādatan, “usually”).
In a more relaxed orthography ة is replaced with ه(hāʾ) with the same pronunciation, especially in Egypt and Sudan.
Loanwords in other languages (e.g. Persian or Turkish) from words with tāʾ marbūṭa use the endings at, ah, a or e.