El árbol murió.

Because árbol is a specific noun here, and in Spanish singular countable nouns normally need an article or another determiner.

• El árbol murió = The tree died
• Just Árbol murió would sound incomplete or wrong in normal Spanish.

Spanish often omits subject pronouns like yo, tú, él, but it does not usually omit articles before nouns like this.

Grammatical gender in Spanish is not always predictable from meaning. While many masculine nouns end in -o and many feminine nouns end in -a, lots of nouns do not follow that pattern.

Árbol is simply a masculine noun, so it takes el:

• el árbol
• los árboles

You usually have to learn the gender together with the noun.

Murió is the preterite form of morir, third person singular.

So it means:

• he/she/it died
• the tree died

The preterite is used here because the sentence refers to a completed event in the past.

The accent mark shows the stress: mu-ri-Ó.

Without the accent, murio would not follow the correct written stress pattern for this verb form. Spanish uses accent marks both to show pronunciation and, in some cases, to distinguish forms.

So:

• murió = stressed on the last syllable

Because the word is stressed on the first syllable: ÁR-bol.

A word ending in a vowel, n, or s normally has stress on the second-to-last syllable. But árbol ends in -l, and by the normal stress rules, it would naturally be stressed on the last syllable unless marked otherwise. The accent tells you the stress is irregular:

• árbol = ÁR-bol

Yes, morir is a stem-changing verb, and it is somewhat irregular.

In the preterite, the stem changes from o to u in the third person forms:

• murió
• murieron

That is why you get murió, not morió.

Yes, you may hear se murió, and it is very common with people and animals. With things like a tree, murió is the simpler and more neutral choice.

In general:

• murió = died
• se murió = also died, often a bit more colloquial or emotionally involved depending on context

For a tree, El árbol murió sounds perfectly natural.

Because murió and está muerto are not the same.

• El árbol murió = the tree died
  This focuses on the event.
• El árbol está muerto = the tree is dead
  This focuses on the resulting state.

So if you want to say what happened, use murió. If you want to describe its condition now, use está muerto.

Yes. The verb form murió is third person singular, so by itself it can refer to:

• he died
• she died
• it died
• you died in formal singular (usted)

The noun el árbol makes it clear that here it means it.

A simple pronunciation guide is:

el AR-bol mu-ri-O

More approximately in IPA: [el ˈaɾβol muˈɾjo]

A few points:

• r in árbol and murió is a tapped Spanish r
• á is stressed in árbol
• ó is stressed in murió
• Spanish vowels are short and clear: e, a, o, u, i, o

Yes. El árbol murió normally refers to a specific tree that both speaker and listener can identify from context.

If you were speaking about trees in general, you would usually say something different, for example:

• Los árboles mueren = Trees die
• Un árbol murió = A tree died

So el here usually means a particular tree, not trees in general.