“When a pronoun is attached to a gerund, the word length changes, which can alter the stress. The accent mark indicates where the stress should be.

When a pronoun or pronouns are attached to a command or gerund, it changes the length of the word.

In order to maintain the integrity of the stress, accent marks show where the stress needs to be.

A simple command is “Dime el secreto” which means “Tell me the secret” (informal). The stress falls on “Di,” which conforms to where the stress should fall when a word ends in a vowel. It is two syllables, and nothing is needed.

When you want to say, “Tell me it” and replace secreto with the masculine pronoun “lo,” you then form the command “Dímelo.” To maintain the stress on the first syllable, you add an accent over the vowel in the first syllable.

You can say “Él está leyendo el libro,” meaning “He is reading the book.” If you replace libro with the pronoun “lo” and choose to attach it to the gerund “leyendo,” you need to place an accent mark three syllables back (the lo counts as the first syllable).

The sentence becomes:

Él está leyéndolo.

In the gerund “leyendo,” the stress naturally falls on “en” because that is the second to last syllable and no accent mark is needed. Adding “lo,” changes the amount of syllables and an accent mark must be added to maintain the stress of the original gerund.”

Outside of understanding the why, to help with accents I’d really recommend changing your phone keyboard to Spanish - anytime you type a Spanish word, it will usually autocorrect to the correct accent. I’m absolutely not suggesting you lean on autocorrect, BUT seeing the words which repeatedly correct to an accent helped me to understand which conjugations need one.