Learn Spanish (Latin America): No he visto la mancha roja.

Visto is the irregular past participle of ver (to see). While most verbs follow a regular pattern of adding -ado or -ido to the stem, ver is one of the common exceptions you just have to memorize for this tense.

Why do we use he for "I have" here instead of tengo?

Tener (to have) is only used for owning or possessing things, like having a car. When you want to say you have done an action, like "I have seen," you must use the helper verb haber. He is the "I" form of haber, and it works together with visto.

Can I put the no between the two verbs, like "he no visto"?

No, in Spanish, you can never separate the helper verb (he) and the past participle (visto). The word no must always go directly before the conjugated helper verb.

Why does the adjective roja come after mancha and end in "a"?

In Spanish, adjectives usually come after the noun they describe. Also, because mancha (stain) is a feminine noun, the color rojo (red) must change its ending to roja to match its gender.