What are the hardest Spanish sounds for English speakers?

The hardest Spanish sounds for English speakers are the rolled rr, the soft b, d and g between vowels, the pure vowels, and the throaty j. None is impossible. Spanish has a small, regular sound system, so a handful of fixes gets you most of the way to a clear accent.

The rolled rr

Perro (dog) and carro (car) need a trill: the tongue tapping quickly against the roof of the mouth. It's the one everybody asks about, so it has its own page on how to roll your R. One thing to know: the single r in pero (but) is a quick tap, like the middle of "butter" in American English. The trill is only for rr and for r at the start of a word.

Pure vowels

English vowels slide. "Go" is really "go-oo," and "day" is "day-ee." Spanish has five clean vowels that don't move:

a like "ah" in "father"
e like the "e" in "met," a bit tighter
i like "ee" in "see"
o like "o" in "more," without the slide
u like "oo" in "moon"

Many learners don't realize this is the biggest thing. Hola isn't "oh-lah" with a long gliding o, and no shouldn't be "noh-oo." Keeping vowels short and pure instantly sounds more Spanish.

Soft b, d and g

Between vowels, b, d and g turn into soft sounds where the lips or tongue barely touch:

Lado (side): the d is close to the "th" in "this."
Haber and lavar: the b is a gentle breath, not a hard stop.
Lago (lake): the g is a soft airflow.

If you say them as hard English stops, people still understand you, but it sounds foreign. This is also something to listen for, because it's one reason native speakers sound fast.

The Spanish j and g before e/i

Jamón, rojo and gente use a sound like a harsh "h," made in the back of the throat. In Spain it's rougher, and in much of Latin America it's softer. English "h" is a good starting point. Just make it a little more scratchy.

Other sounds that surprise people

ñ is like "ny" in "canyon": año (year), mañana (tomorrow).
ll and y are pronounced in different ways depending on the country: like "y" in "yes" in many places, and like "sh" or "zh" in Argentina and Uruguay.
z and c before e/i sound like "s" in Latin America and like "th" in much of Spain.
h is always silent: hotel sounds like otel.
v sounds the same as b. Yes, really.

How to work on them

Pick one sound per week. Trying to fix everything at once makes you tense.
Listen and copy. Slow audio is useful for hearing the real sound before you try it.
Record yourself and compare to a native recording of the same phrase.
Use minimal pairs, like pero and perro, to train the difference.

For the listening side, Langtok, which I made, lets you slow short Spanish stories down to 0.25x while each word highlights, so you can hear how sounds like rr and the soft d really come out in speech before you try them yourself.

Remember that perfect sounds aren't the goal. Clear sounds are. An accent doesn't matter that much, as long as people can understand you.