The first sentence is incorrect. It should be: Mahal ang librong ito. / Ang libro na ito ay mahal. This book is expensive. Mahal na libro ito. / Ito ay (isang) mahal na libro. This is an expensive book. Simply put, "ang" is equivalent to the English article "the". In the first sentence's case, it becomes "this" because of "ito", specifying which book you are talking about. A literal translation would be, "The book that is this is expensive." :)) Additionally, you may hear a lot of people say, "ADJECTIVE itong NOUN na ito." It's redundant, of course, but it's common usage. Perhaps it's for emphasis. >> How do I decide which words to connect with "ng" or "na"? If the preceding word ends with a vowel, you append "~ng" to the end of the sentence. If the preceding word ends with a consonant, you use "na." This applies to both adjectives and nouns, but they may have different nuances. Magandang babae / Babaeng maganda Beautiful woman Ligaw na bata / Batang ligaw Lost child Itim na sapatos / Sapatos na itim Black shoes Counters are an exception. For example, you cannot interchange "apat na mansanas" (four apples) with "mansanas na apat."

There are various way to say it: Ang librong ito ay mahal. ~ sentence structure (Ang = The) which is a definite article + Noun (libro = book) + pronoun (ito) + (ay) often translated into English as 'is' or 'are' or 'am.' + adjective (mahal). Mahal ang librong ito. ~ (Adjective mahal) + (definite aritcle "ang") + Noun (libro) + pronoun (ito). Mahal na librong ito. ~ (Adjective mahal) + (na) is a linker to the object + (object "libro") + (pronoun "ito). "Ang" is usually used at the start of the sentence, and is translated in English as "the" which is a definite article. So sometimes, it is ommited since the message of the sentence clearly states it, and that is the "book" How do I decide which words to connect with "ng" or "na"? Usually, "NA" is used after the adjective that ends in consonant: mahal "na" libro ~expensive book makapal "na" libro ~thick book manipis "na" libro ~thin book and "NG" is used after the adjective that ends in vowels (you connect it to the adjective): maganda+ng = magandang libro ~pretty book, beautiful book, good book luma+ng = lumang libro = old book bago+ng = bagong libro + new book puti+ng = puting libro = white book libre+ng = libreng libro = free book technically, you can use "NA" to adjectives that end in vowels, but it will sound awkward, and make the sentence rather long, so you have another reason to use "NG" instead, :) SO, that's it, i hope this will help you.

Yes, "ang" corresponds to "is". It is used if the subject you are pertaining is singular. Your second sentence should be written in this way, "Ito ay isang mamahaling aklat." The connector "na" is used if the preceding word ends in a consonant. Ex: Umakyat siya sa mataas na bundok. (He climbed the high mountain.) While, the connector "ng" is used if the preceding word ends in a vowel. Ex: Ang mga bata ay masayang naglalaro. (The kids had fun playing.) ~ Hopefully this will help you :)

Thank you very much to all of you for taking the time to answer! Truth be told, my Tagalog is nearly non-existent. I'm learning some basic vocabulary through a course on Memrise, which I'll continue for the next few months, and then I'll jump into real study in May. I'm going to save your responses so that I can read them again later on, when I'm more experienced and can better appreciate your words of wisdom. Salamat!