Until now we've learned two rules for making a Spanish noun plural:
- If the noun ends in a vowel, add -s
- If the noun ends in a consonant, add -es
The second rule has some exceptions, which are summed up below.
If the noun ends in a z, drop the z and add -ces
If the noun ends in an s or an x, add -es only if the stress is on the last syllable (which is always the case in monosyllabic words).
Otherwise, add nothing.
And finally, if the noun has an accent mark on the last syllable, we remove the accent mark if that doesn't influence its pronunciation.
This is because the stress is on the second last syllable by default, so here aviones retains the stress on on even though we drop the accent mark.
Note that in países we didn't drop the accent mark, since this would have changed the pronuncation, because without the accent mark país would have been pronounced as one syllable instead of two.