Learn Spanish (Latin America): Yo tengo calor y decido abrir la ventana. - I am hot and I decide to open the window.

In Spanish, you use the verb tener (to have) for certain physical feelings like being hot, cold, hungry, or thirsty. So, instead of saying "I am hot," you literally say "I have heat" (tengo calor). If you use estar and say estoy caliente, it usually means something completely different (often meaning "I am feeling sexually aroused" or having a physical fever)!

Why does decidir end in an "-o" here (decido)? I thought it was an "-ir" verb.

You are right that the base form is decidir (an "-ir" verb). However, in the present tense, regular "-ar", "-er", and "-ir" verbs all share the exact same ending for the "I" (yo) form: they all drop their ending and add "-o". Because this is something "I" decide, it becomes decido.

Why is abrir left in its dictionary form instead of being conjugated?

When two verbs go together like this, usually only the first one gets conjugated to match the person doing the action. The first verb is decido (I decide), and the second verb stays in its infinitive (dictionary) form, abrir (to open). This matches English perfectly: "I decide to open".