Verified answer

As a literal translation it would be right, but it sounds a little awkward. I'd say "I become sad when I think about it"---"Me pongo triste cuando lo pienso" And even then, the "lo" sounds awkward too (though not incorrect if you are referring to an idea, like "lo" being "mudarme a otro pais" for instance (move to another country) instead of an object like "arbol") I don't know if you still follow the argument of this native speaker but I'd say "Me pongo triste cuando pienso en ello/eso"

Yo estoy triste cuando pienso en eso.

Yeah, but it would sound a little better if you'd say " Me pongo triste cuando lo pienso,". But the first one is good.

The most accurate translation would be:

Estoy triste cuando pienso en/sobre ello.

But the meaning is almost the same. So, your translation is ok.

For me it's ok.

You could also say.

ESTOY TRISTE CUANDO PIENSO ACERCA DE ESO.

Perfect.

Other users suggested "me pongo triste..." which is ok too, but that means "I GET sad..." not I AM. ☼

I get sad when i thinking about him