I have written many times before on how German (as a Germanic language) is more closely related to English than any Latinate language (like French, Italian or Spanish), and is indeed fundamentally the same. In some ways, this makes it easier to learn.
However, much though professional linguists will dispute my claiming this so definitively, the fact is German is a harder language to learn than Spanish for the average English speaker. How and why?
Consider the Spanish phrase:
con el perro
Here, my core vocabulary as even a novice would tell me that “con” is a preposition meaning something like “with”, and “el” is an article, “the”, marking masculine singular in this case (as, like most Latinate languages, Spanish distinguishes between two genders, masculine and feminine). We may also know, or be able to work out from the context, that “perro” in most instances means “dog”.
The advantage with Spanish is we now know not only what the word “perro” means but also how to use it. Nearly all words ending in –o are masculine and the plural in Spanish is formed by -(e)s, so we not only know that “dog” is “perro” but also that “dogs” is “perros“. This is the same regardless of the use of the word (whether it is a subject, and object, comes after a preposition, or whatever).
If we turn to German, life suddenly becomes a lot more complex.
mit dem Hund
For similar reasons to the above, we can work out that this means “with the dog”. We know from this what it means, and in particular what the word “Hund” (cognate with English “hound”, to make things even easier) means. However, we have a problem – we still have no idea how to use the word!
Firstly, even the article “dem” tells us only that “Hund” is masculine or neuter (German nouns have three genders, unlike in any other major Western European language). Secondly, worse still, we have no idea what the plural form is – it could be “Hund“, “Hünd“, “Hunde” (which is fact it is), “Hünde“, “Hunder“, “Hünder“, “Hunden“, conceivably “Hünden” or maybe even “Hunds“. This may be before we have come to learn that the dative plural (German also has four cases, two of which in the modern spoken language may be used after prepositions) generally adds –n – so, notwithstanding the above, the plural form would actually be “Hunden” in this case (literally!)
The immediate difficulty with German, therefore, is that it is not as easy to “absorb” in a way which means you can then use it accurately. Spanish has a much clearer and simpler set of markers than German has, making it more instantly accessible to learners.
This is not to say that Spanish is straightforward. The average verb in Spanish has over 50 distinct forms (invariably approaching 40 in common use), compared to just four in English and six in German. The point is, however, that once the patterns and irregularities are learned, they are clear; whereas in German, particularly with nouns, there are simply fewer reliable patterns and things like gender or plural form just have to be learned individually (even if some can be reasonably guessed).
That is the “how”. What about the “why”?
The reason that German has been more conservative with nouns and less so with verbs than Latinate languages such as Spanish (and indeed more conservative than other Germanic languages generally) is not easy to determine.
Broadly, German is a more noun-based language, which may explain why it has retained its complexities predominantly around them (effectively retaining only partially predictable “noun classes”), while simplifying verbs.
Nevertheless, there is no clear reason why German is quite so conservative, even versus similar languages such as Dutch. It was not a deliberate ploy around the time of standardisation (as it was for Italian), nor has German been particularly isolated (like Icelandic).
That German is tougher to reproduce accurately than Spanish for English speakers despite its closer family links may simply by luck of the linguistic draw.