The one version of the verb still missing. For -ar verbs, take off the -ar and put -amos, exactly the same move you have been making all along.
tomar → tomamos · hablar → hablamos
With -amos you are adding two syllables, so the accent, still on the second-to-last syllable, sits in a different place from the other versions. Run through tomo, toma, toman, tomas, tomamos and you can hear it moving.
Hablo · Habla · Hablan · Hablas · Hablamos
This follows from the whole point of the splitting rule: the vowel splits because you stress it. In the "we" form you no longer stress it, so it stays intact.
Muestro but mostramos · Pienso but pensamos · Cierro but cerramos
Now that you have every version, you can run a splitting verb right across the set.
¿Qué quieres? · Lo cuelgan · Los cuelgan · Los cuelga · Pienso que sí (I think so) · ¿Qué piensas hacer? (what do you plan on doing)
There is no way to know for certain which verbs split. Comer gives como, no split. Most of the time it will split, so experiment and you will always get feedback. You will also develop an intuition for it.
English words that begin al-, as opposed to ending -al like normal or metal, come from Arabic. That al- means the in Arabic, and it is written joined to the word, so words arriving from Arabic mostly came in carrying it.
So when you say the alcohol, you are saying something like "the the alcohol".
In Arabic the l of al assimilates to match the next letter when the two are close, which it does with about half the letters. So al zucar became azúcar, and al ruz (the rice) became arroz. English took sugar and rice without the the, which is why they look so different now.
Two s's together are not a combination you find in Spanish, which is why azúcar takes a z. English possible and passion come across with one s.
Double r does exist, precisely because it differentiates the normal r from the rolling r.
Tomar is used generically like take but also for drinking, and it is extremely common. Beber exists but tomar is what you will hear.
No tomo alcohol · No tomo azúcar · Quiero arroz · ¿Qué tomas? (a way of asking what you want to drink)
Cerrar sounds /therrar/ in most of Spain. De- in demostrar means of in Spanish.