How easy/hard is Filipino/Tagalog to learn?
The Foreign Service Institute (US Department of State) classify it as:
Category II: Languages with significant linguistic and/or cultural differences from English 44 weeks (1100 class hours)
(See the table to compare it with other languages)
My own thoughts are: that anyone that knows Spanish has a small head start with the vocabulary; and that good resources are quite hard to find.
In my experience, getting to the point where you can understand well when you hear it isn't very hard, but reading, writing, and speaking are a different story. Then again, my ear is already trained because half of my family is from the Philippines and I can get pretty far on context clues.
Spanish speakers will have a small advantage in vocabulary because Tagalog has a lot of words that come from Spanish. However, they can be pronounced and spelled differently because Spanish and Tagalog use slightly different alphabets*. For example, the Spanish cuchara, tenedor, and cuchillo are kutsara, tinidor, and kutsilyo in Tagalog. Also, in many situations, Tagalog has a Spanish loanword but another word (or several) of more native origin with the same meaning but they're not always completely interchangeable. Tagalog speakers can understand much more Spanish than Spanish speakers can understand Tagalog.
The hardest parts are sentence order and verb conjugation. Generally sentences are formed with the verb coming first, which is hard for most European language speakers to get used to. Verb conjugations are very complex. Unlike languages like Spanish where you conjugate verbs by changing the ending, in Tagalog you conjugate verbs by adding onto the beginning, middle, or end of the verb depending on certain factors. Sometimes you add onto more than one part of the verb to conjugate it, which can result in a very long word that looks absolutely nothing like another conjugated form of the same verb. The factors that determine how you conjugate verbs are concepts that I'm told are sometimes difficult for even native speakers to grasp. Instead of tense (past, present, future), the progress/completion of the verb is what matters (whether it's not yet started, in progress, finished, or just now finished). Trigger and mood also determine conjugation, and can't be explained in few words.
Pronunciation is fairly simple and consistent. As a heads-up, when two vowels are next to each other, you pronounce both of them separately. "Tao" (meaning person) is pronounced "ta-o." "Bataan" (as in the location of the Death March of Bataan) is pronounced "ba-ta-an." Properly written Tagalog has accent marks indicating where to put emphasis and what kind of emphasis, but native speakers don't actually use them even in some published documents, so you pretty much just have to memorize where emphasis goes.
Resources for learning Tagalog tend to be very basic and of low quality for a number of reasons, and one of them is the arbitrary distinction between Tagalog and Filipino. As I understand, Filipino is the official language of the Philippines (along with English), it's standardized, and it IS Tagalog as it's spoken in the Manila area. However, Tagalog spoken in other areas is broader and less standardized. Many people alive in the Philippines today were around before Tagalog was chosen as the national language, and decades after that official documents started calling it Filipino, so many Filipinos still say that Tagalog is the language of the Philippines. For the record, the first time I heard a Filipino refer to the language as Filipino instead of Tagalog was less than 10 years ago. Some resources take a broad Tagalog approach while others take a strict Filipino approach, which leaves out a lot of what you'll hear from natives. Plus, teaching the differences in grammar is hard to do using English as the starting language, particularly when there's no way for the resource to give the learner feedback. They also have to account for a lot of synonyms since there are so many loan words and pre-existing native words that mean the same thing. Take a look at the incubator page for English for Tagalog speakers and scroll down to 9-10 months ago and you'll have a brand new respect for language contributors.
There aren't many resources for learning Tagalog because there isn't very high demand. It's spoken almost only by Filipinos, the majority of whom speak English and often even more languages. Most of the Filipino-Americans I know who grew up in the US don't speak Tagalog either because they have an American parent and therefore English is the common language in the family, or because their parents didn't see any need for them to learn the language. I know several Fil-Ams who are trying to pick it up, but the general sentiments I've heard are that anything beyond an intermediate level is unrealistic without intensive months-long immersion.
I've done relatively little formal learning in Tagalog, but I've been around people speaking it all my life and I've spent several summers in the Philippines. Yet, I'm still uncomfortable speaking and writing anything more than simple sentences. Every now and then, I see text written in another language of the Philippines or even Malaysian and it takes me a moment to figure out that it's not Tagalog. It's a fairly low priority language for me so for now I'm happy with my family's English grammar-based Taglish.
*I've read conflicting info about alphabets. My understanding is that the Tagalog (therefore not standard) alphabet doesn't include several of the letters that the Spanish and English do, one of them the letter "C," which is why the Spanish cuchara and cuchillo become the Tagalog kutsara and kutsilyo, and the missing letters are only ever used in foreign words and names. However, I have resources that say that the Filipino alphabet (which is standard and published) includes all the letters of the English and Spanish alphabets plus more. Yet, those letters are still only used in foreign words and names so it seems that this alphabet is just the Tagalog alphabet expanded for certain accommodations.
How regular is Tagalog? That'll significantly determine difficulty of the language. So from what you say about conjugation, it's an agglutinative language like Zulu and Xhosa. What are the other parts of speech like? Noun plurals, adjective agreement, pronouns. The syntax sounds like Biblical Hebrew, VSO.
Not sure how hard, but as a Spanish speaker, I'd probably have an advantage.
It depends on your native language tho. As a native speaker, having basic knowledge in Spanish could only help you in vocabulary (but not that much, it's just like having a basic knowledge in English helps one in learning French.) But French and English have some similarities (there are differences too) tho in terms to grammar unlike Filipino and Spanish.
Reading, and speaking Filipino words is just easy. I noticed that some foreigners have a hard time in pronouncing "ng" sound, that's it.
Verb conjugation and sentence structure may be a bit difficult for foreigners tho.
Kumain ako ng pating. Kumain (past tense/completed form doer focus) ako (pronoun used as a subject) ng (object marker in this case) pating (shark) I ate shark
Kinain ako ng pating. Ate (past tense/completed form object focus) ako (pronoun used as a subject) ng (doer marker in this case) pating (shark) The shark ate me.
The meaning changed just because of -um- and -in- infixes. You'll get used to it tho if you try your best