"Never write the letter "Я" after the letters 'Г, К, Х, Ж, Ч, Ш, Щ, Ц' instead use "А"" http://www.russianlessons.net/grammar/spelling_rules.php
Took me a while to figure out, but the accusative plural of inanimate direct objects uses the nominative plural. Nominative Neuter Singular "towel" is полотенце. According to the declension tables, the ending е should change to я BUT under the Russian Spelling Rules, я after Ц has to be changed to а, resulting in полотенца.
For my own purposes, I reorganized the table of spelling rules so that, rather than doing a look-up on what the results of the change are (which letters get changed to which), I could first see which Russian letters required a change, then look up the change. I find this more convenient. Here is my chart:
Ж Ш Щ....ы - и, о^ - е, я - а, ю - у
Ч Ц.......................о^ - е, я - а, ю - у
Г К............ы - и............. я - а, ю - у
Х..................................................ю - у
^Unstressed “o” only
I'm almost 100% sure I made the transpositions correctly, but if anyone sees an error, please let me know.
These are the first person singular possessive pronouns (my/mine) for the three genders:
- Мой is the masculine pronoun used with masculine nouns as in "мой нож" (my knife)
- Моя is the feminine pronoun used with feminine nouns as in "моя вилка" (my fork)
- Моё is the neuter pronoun used with neuter nouns as in "моё окно" (my window)
From the other words in the sentence. If it's plural then adjectives, possessive pronouns and so on are also in plural form.
In "мои полотенца" you know that "полотенца" is plural because "мои" is plural also.
If it were something like "моя фирма" then you'd know right away "фирма" is singular because "моя" is feminine singular.
If it were "белые полотенца" ("white towels") you would know that "полотенца" has to be plural form because "белые" is also plural form.