It is a very good question. The oo and o in hoort and horen are exactly the same, as well as the ee and e in speelt and spelen.
In dutch the distinction between ''short'' and ''long'' vowels is very important.
In fact, the rules for whether something is a short or long vowel have some similarities with English. Compare the pronounciation of the vowels in ''far'' vs ''fare'', ''fact'' vs ''face'', ''is'' vs ''ice'', and so on.
So I"ve listed the rules that you can use to assess whether a sound is short or long below! I suppose there are some exceptions and special cases, but the rules are applicable in most situations. At the end I'll put some examples for you.
<> cases in which it's short
*1* - When the vowel is followed by only one consonant, at the end of the word.
*2* - When the vowel is followed by two or more consonants (regardless of the position in the word!)
*3* - The vowel is followed by a consonant and then another vowel
*4* - The word ends in a vowel, but not the ''e'' (which is always short as the final letter of the word)
*5* It's a diphtongue or consists of two vocals
''spEElt'' is long because it's *5* (two vocals) and ''spElen'' is also long because it's *3* (vowel-consonant-vowel).
*1* bEd, gEk, wAt, watEr, mAn, zevEn, zEgt, hooivOrk, hebbEn, hEt
*2* wAndelen, pOtlood, zevEntien, jUllie, bIdden, pAkken, breedtE (as of rule *4*), hEbben
*3* rEden, zEven, gEven, lOpen, spElen, tAfel
*5* gOOien, gOUd, brEED, brEEdte, hUIs