Difficult diminutives

Question

Diminutives are easy to get a hang of for relatively simple nouns: add -inho / -inha if ending in -o / -a or -s or -z (carr|inho, Japones|inho) or -zinho / -zinha otherwise (cafe|zinho)

But I'm not sure how form diminutives for words, which are somehow "irregular", e.g.

• masculine nouns ending in -a
• feminine nouns which irregular stress, or
• words, which end in something already containing -nh-.

Which of the following would be correct (or most common)?

• idioma (or poeta or problema)
  o idiominha¹, a idiominha², o idiominho³, o idiomazinho⁴
  _{¹ because idioma is masculine, but ends in -a
  ² because the word ends in -a and the ending -inha is feminine
  ³ because idioma is masculine, and the ending -inho/a has to match gender
  ⁴ because idioma is too irregular to use -inho/a}

• língua
  a linguinha, a linguazinha

• lenha
  a lenhinha, a lenhazinha

• mandioquinha (not a small mandioca, but rather this)
  a mandioquinhinha, a mandioquinhazinha

Answer 1

I couldn't find the rules in the literature, so this is my answer as a native speaker from Portugal. A few things below might not apply in Brasil.

All the cases you referred use the diminutive version -zinho/a.
In these cases the root word is unchanged, that is, no ending letters are dropped, as is the case with the suffix -inho/a.

• Masculine nouns ending in -a : use the suffix -zinho (but see below)

problema - problemazinho
poeta - poetazinho
um jornalista - jornalistazinho
uma jornalista - jornalistazinha

However, in Brasil, you can hear the version probleminha, which is not heard in Portugal.

• Following diphthongs : use the suffix -zinho or -zinha

língua - línguazinha
coração - coraçãozinho
rádio - radiozinho

Note: comboio - comboinho
(perhaps because the last o is outside the diphthong)

• Nouns ending in -nh- : use the suffix -zinho/a

moinho - moinhozinho
caminho - caminhozinho
façanha - façanhazinha

Note:
castanho (adj) - castanhinho
castanha (noun) - castanhinha or castanhazinha

Note that some nouns and adjectives can accept both forms of diminutive, most commonly in words related to food:

fruta - frutinha or frutazinha
carne - carninha or carnezinha

Be also aware of the uses of diminutive forms. At least in Portugal, they are preferred when:

• speaking to children
• talking about food or shopping food
• in situations where we mean to be ironic.

Of course, they are used to refer to things that are small, but there are many words of which I have never heard the diminutive form, and couldn't tell how they would sound like.