Let’s tackle pronunciations using named places. If you say it gringo-style, they might still understand you, but to really sound like a local, you’ll have to work at it a bit…
Let’s start with one of the most frequently mispronounced city names: Medellín. See that accent on the i? Anytime you have an accent on a vowel, it emphasizes that vowel’s sound. Now, in Spanish, that double l is actually a letter in the alphabet and its pronunciation can vary according to where you are in Latin America. In Argentina, it’s a strong SH sound, so “callate” (shut up) sounds like cash-a-teh, but lucky for you, in Colombia it’s more like an English Y with a very slight SH in it, so callate sounds like cah-yah-te. That said, Medellín sounds more like Med-eh-JEEN with a slight emphasis on the JEEN. It’s not a J, but a cross between a J and a Y, which will be hard for any gringo to deliver, so if you do a soft J (not like the DJ as in Jeans), you‘re closer to the real sound than if you say Med-eh-YEEN.
As mentioned above, accents on vowels mean you have to emphasize the sound of that vowel, so Bogotá is Bo-go-TAH, where you slightly emphasize the TAH without screeching it out! English news reporters usually get it wrong making it sound like Bogotaw as in thaw, so you can correct them now!
The umlaut (two dots) on the U is also something you have to mind. Let’s take Itagüí. Not only does it have the U umlaut but the last i has an accent, so it’s pronounced Ee-tah-goo-EE. You emphasize the i. If there was no umlaut, it would sound like Ee-tah-guEE, which might not be understood by locals!
Another sound that’s difficult not only for English but French and other language speakers is the double R. The single R is already more rolled in Spanish than in other languages, but the double R is rrrrolled without exaggerating too much unless you’re a football announcer! Let’s go all out on this one with Valle de Aburrá as our named place. There’s a V at the beginning which we’ll tackle in the next paragraph but also a double R and an accented A, so what it should sound like is Bah-yeh deh Ah-boo-rrrr-AH. Vibrate that long R!
The V in Spanish is more like a soft B, so in the prior example, Valle de Aburrá, the Valle sounds more like bah-yeh. If you meet a Verónica you’ll call her Beh-ROH-knee-cah or she might teasingly correct you!
Another misunderstanding is the Y in Spanish. While in Colombia the ll is like a Y, the Y itself is closer to how most Latin Americans pronounce it, except for Argentinians who again SH it (no pun intended): it’s closer to a soft J (a J sound minus the D sound before it). So Ayurá sounds like Ah-joo-RAH but not Ah-djoo-rah.
If you think this is hard, remember that in Spanish what you read is what you’ll say, whereas in English you can have the exact same word pronounced 3 different ways and with 3 different meanings. In other words, Spanish is actually much simpler for an English speaker to learn than English is for a Spanish speaker since so much of the English language is non-intuitive, even if our grammar is much simpler. Don’t make me trot out name places like Worcester (pronounced Wooster) as examples!
It’s not easy to transmit sounds through writing, therefore you might want to use one of the many websites that have sound clips alongside word definitions, but do mind the fact that pronunciation varies regionally, so that Spaniards sound nothing like Colombians!