Learn Spanish (Latin America): Yo nunca había sentido algo tan abrumador. - I had never felt something so overwhelming.

Había sentido is the past perfect tense (often called the pluperfect), meaning "had felt." You use it to talk about an action that happened before another point in the past. Here, the speaker is looking back from a specific past moment and saying that, up until then, they hadn't felt this way.

Can I put nunca after the verb instead?

Yes, but Spanish requires a double negative if the negative word goes after the verb. You would have to say Yo no había sentido nunca algo tan abrumador. Putting nunca before the verb is very common and lets you drop the no.

How does abrumador work, and can I use it to say I am overwhelmed?

Abrumador means "overwhelming" (describing the thing causing the feeling). If you want to say that you are overwhelmed, you use the past participle abrumado or abrumada with the verb estar or sentirse. For example: Me siento abrumado.

Why use tan instead of muy here?

Tan translates to "so" in this context, giving it an emotional emphasis: "something so overwhelming." If you used muy, it would mean "something very overwhelming," which is grammatically fine but loses that dramatic "so" flavor.