[DonaldPShimoda]

It's more complex. Spanish phonology dictates a denti-alveolar pronunciation of /t/, while typical English phonology (USA, England, etc.) uses purely alveolar /t/. So in Spanish, the tip of the tongue hits the back of the teeth, which doesn't happen in English. This distinction is not rendered in the IPA glyphs or any diacritic that I'm aware of.

[thaumasiotes]

Is it not the t̪? https://en.wikipedia.org/wiki/Dental_consonant

[DonaldPShimoda]

I think it's not perfect, because (I believe) Spanish /t/ is usually denti-alveolar and not fully dental. I don't think there's a way to express that specific aspect of it, though I'm also not a linguist so I may be mistaken!

[leephillips]

That’s what I’m talking about. The Spanish t is almost like an unaspirated English th.

[DonaldPShimoda]

Aspiration is a separate phenomenon. English also has unaspirated plosives — they just mostly only occur in word-final positions, like the <t> in "pat", and they are not contrastive with their aspirated versions.

What I was talking about was how the Spanish /t/ has the tip of the tongue just a bit further forward, so that it touches the back of your teeth. English /t/ is, for most people, purely alveolar, so there's no teeth contact.

[leephillips]

Agreed. But some Spanish speakers pronounce t with the tongue quite far forward, between the teeth; that’s what I meant by comparing it to an unaspirated English th. But I think we are getting into regional variations now.

It’s even more crucial with d. An English native speaker who is a beginner in Spanish will pronounce tened in a way that will be interpreted by Spanish speakers as tener.