Some wonderful mnemonic devices I picked up in a different thread:
1) Big 3 (der, die, das) get an 'e' (nominative). Example: das neue Buch.
2) Plurals and changin' gets an "en": Accusative masculine, all plurals, and datives have their adjectives ending in '-en'. Example: die neuen Bücher.
3) No big 3? Article takes over. Example (combines 1 and 3): dieses neue Buch.
Forgive me if I screwed something up. I'm still getting them figured out myself. They've been a massive help though.
This is an example of weak declension. The adjective ending is determined in part by the article/pronoun, but their endings wont necessarily match.
Easiest way to think about the inflection in this example (and any other adjective inflection) is as follows:
The GOAL/AIM/OBJECTIVE of adjective inflection (in my opinion) is for any speaker (native or non-native) to be able to work their way back from the noun itself to its underlying grammatical gender. For instance, I am learning German, like the rest of you. If, while reading, I come across a noun I have never seen before and therefore have absolutely no idea what it means, WOULD I STILL BE ABLE TO IDENTIFY ITS GENDER? If yes, the inflection has been done properly.
Beispiele (assume I did not know what 'das Buch' meant):
1) Ich habe das neue Buch gelesen ("I have read the new book"). Here, 'das' clearly marks the gender of 'Buch,' and therefore 'neu' only needs an -e ending (the so-called "weak inflection." You will notice this is the same concept that applies in this particular DL example. 'dieses' identifies the neutral gender of 'Buch,' therefore 'neu' does not need to, and thus only adds an -e.
2) Heute habe ich ein neues Buch gekauft ("Today I bought a new book"). Why is it 'neues' now? BECAUSE 'ein' FAILED TO IDENTIFY THE GENDER OF 'Buch.' I have no idea (refer back to above assumption) what 'Buch' means, so has 'ein' at least identified the gender for me? No. In my ignorance, 'Buch' could be masculine or neuter. Therefore, 'neu' has to inflect to show that 'Buch' is a neuter noun, thus 'neues.' You will notice the '-s' ending of 'neues' and 'das' both match.
Personally, I use this thought process for adjective inflection. I would recommend using the inflection tables first, but honestly, seeing examples is the best practice. You will cry yourself to sleep trying to memorize inflection tables. You are better off having epiphanies while actually working examples, and will thus retain it better.
@JonGudmundson was getting at this above, I simply provided the long-winded explanation.
Ich hoffe, meine Erklärung hilft euch :)
I mean that when you are to conjugate an adjective, (at least when they teach you that in Spain) there are three situations: after "artículo determinado" (der, die, das, die), "artículo indeterminado" (the rest) and "without article" Source in spanish: http://es.wikibooks.org/wiki/Alem%C3%A1n/Gram%C3%A1tica/Adjetivos
If we conjugate neu considering "dieses" the same as "das", it is neue, but if it is not the case it should be "neues", am I wrong?
I see. You meant "definite articles". Demonstrative pronouns are in the same group (weak inflection) as definite articles.
So, if I'm understanding this correctly, "neue" is declined with a weak inflection because it follows "dies-", but "dieses" is declined with a strong inflection?
In other words, not only do you have to keep in mind the number, grammatical gender and case for a given clause, but words WITHIN THE SAME CLAUSE can be declined differently? Is that an accurate statement?
Dieses is not declined, it simply has a dedicated form for each gender (dieser, dieses, diese, diese). You wouldn't say das is declined. Since it's in its original form, it clearly shows the gender of the book, so the article doesn't have to: hence the -e ending.
I don't recommend using this weak inflection thing, it's quite a brainless exercise to memorize endings. It's an artificial thing that's made up to help teaching children, and it's quite disconnected from the actual mechanics of the language.
No, "dies-" follows weak declension, along with the definite articles ("der" words). The grammatical case determines the article or demonstrative pronoun's declension, and the article or demonstrative pronoun's declension determines the adjective's declension.
Consult these tables: http://en.m.wikipedia.org/wiki/German_declension#Attributive_adjectives
Here is another good article. See the asterisk at the bottom of the table. https://deutsch.lingolia.com/en/grammar/adjectives/declension