Hi cubibobi,
Citta is the “root” form of the word. Root forms do not have plurals. Technically it’s incorrect saying that cittā is the plural of ‘citta’, rather cittā is the plural cittaṃ. ‘Cittaṃ’ is the nominative singular of the root word ‘citta’. So cittā is A plural of ‘citta’ but not THE plural of ‘citta’. Cittā thus is the plural of the singular ‘cittaṃ’ (also the plural vocative, but that’s another story).
The root forms of the words in Pāḷi are rarely used on their own in the Tipitaka (if at all), but are very widely used in Western translations when talking about the concept or base meaning of the word.
By the way, the same applies to ‘sutta’ which is the dictionary form (root form) of the word, where the nominative singular would be suttaṃ and plural suttā.
But both of those root words (citta/sutta) have other plurals, that is – plural forms in other grammatical cases, for example cittāni, suttāni which are both plural forms of “citta” and “sutta”.
Another neuter word which “behaves” like citta and sutta is “mūla” which becomes mūlaṃ in nominative singular but the preferred nominative plural is mūlāni, for example “tīṇi akusalamūlāni” means “(the) three unwholesome roots” (of course those are lobha, moha, dosa, where all 3 of these words too are ‘root forms’, in nominative singular they become lobho, moho, doso).
Also there are really many word forms which can end in long a (ā) in BOTH singular and plural nominative and which are feminine gender, for example “paññā”, “mettā”.
My point is, if you really want to dig deeper into Pāḷi then you must learn its grammar. But it might be not easy for people who don’t speak languages with a case system.
And finally, Sinhala is a great language, but keep in mind that even tho it shares similarities with Pali it’s not the same language in many regards (starting with pronunciation).
If you told me what your native language is I could try to explain things better for you based on it. Lal’s called you Lang, so I’m guessing it must be a sinitic language.