This is a pretty complex, multifaceted question, and you have to first understand the nature of transitive and intransitive verbs. A transitive verb takes a direct object, while an intransitive verb does not take a direct object. For and intransitive verbs - which are conjugated with
essere in the present perfect - the past participle of the verb agrees with the subject of the sentence in gender and number. This agreement rule also applies to reflexive verbs (like mettersi) which are also conjugated with essere in the present perfect.
In this case however,
portare is a transitive verb. It takes a direct object, and you can tell this because transitive verbs are conjugated with
avere in the present perfect. The past participle of a transitive verb will generally not agree with the direct object or indirect object as is the case here.
There are exceptions to the rule though. For example, consider
- L’ha trovata ("He found her/it (feminine).") and
- Li ho avuti. ("I had them (masculine, or a mix of masculine and feminine).")
Thus, if the pronouns lo, la, l', li, or le are used with a transitive verb, the past participle also agrees with the pronoun in gender and number.
Wow, that took a long time to type out. Hope this helps!
I'm not sure that "I looked AT HER" is transitive, because "her" is an indirect object. A transitive verb takes a direct object: ask "who" or "what" the verb did. In "I gave her a letter", "a letter" is the direct object because it is what was given, while "her" is the indirect object - which can usually be expanded into a prepositional phrase - "I gave the letter to her". "Laugh" is a good example of an intransitive verb, because you would laugh AT someone or something. That's how it works in English anyway!