The noun il muro has two correct plurals: i muri and le mura. This is an example of plurale doppio, i.e. double-plural.
They are used in two different contexts.The feminine version of the noun is used the most often, and refers to “walls” in the sense of the entity that is around/supporting something. It also the preferred noun when referring to historical structures:
Le antiche mura etrusche (The ancient Etruscan walls)
Siamo davanti alle mura del centro storico (We’re standing in front of the walls around the old town centre)
Le mura di casa (The house’s walls)
The masculine version is used when referring to walls in the structural sense of the word, for example:
I muri portanti (The supporting walls)
I muri doppi (The double-walls)
Non era mia intenzione is an offer of apology and/or remorsefulness, and it generally means “It wasn’t my intention” for example:
Mi dispiace, non era mia intenzione di svegliarti / spaventarti / urtarti ecc
(I am sorry, I didn’t mean to wake you / scare you / bump into you etc).
On the other hand, Non era la mia intenzione has a very different meaning. It generally precedes the word questa, and it means “This wasn’t the purpose of my actions”, i.e. “I didn’t intend for things to work out this way”.
Mi dispiace, non era mai stata la mia intenzione di ammaccare la tua auto!
“I’m sorry, I never meant to dent your car!”
The sentence structure is incredibly similar between the two phrases but their meaning his very different!
When you’re referring to your parents formally, i.e. mio padre and mia madre, the determinative article is never used.
This also applies to other family members, e.g. mia sorella (my sister), mio fratello (my brother), mio cugino (my cousin), mia zia (my aunt), unless the noun is modified or is followed by an adjective e.g. il mio fratellino (my little brother), la mia sorella maggiore (my big sister), il mio cugino sardo (my Sardinian cousin).
You also use this article when using the informal nouns “papa`” and “mamma”, i.e. il mio papa` and la mia mamma.