Learn Portuguese (Portugal): O pai está muito cansado.

Está comes from the verb estar, which we use for temporary states and conditions. Since being tired is a temporary feeling, we use está. If we used é (from the verb ser, which is for permanent traits), it would imply the father is a permanently tired person by nature!

Why does cansado end in -o?

In Portuguese, adjectives must match the gender of the noun they describe. Because o pai (the father) is masculine, the adjective takes the masculine ending -o. If we were talking about the mother (a mãe), it would change to the feminine form, cansada.

Would muito ever change to muitos or muita in this type of sentence?

Not when it means "very"! In this sentence, muito describes how tired he is, making it an adverb. Adverbs never change their form. Muito only changes to match a noun when it means "much" or "many" (like muitos carros for "many cars").

Why is there an accent mark on está?

The acute accent tells you to put the speaking stress on the final syllable (es-TAH) and gives the "a" an open sound. It is also crucial for telling this verb apart from the word esta (which means "this"). Without the accent, the stress moves to the first syllable (ES-ta).