For higher levels: /e/ in contact with the trill rand /x/ as in perro (dog) and lejos (far) is more open than in other contexts.
A characteristic feature of all five Spanish vowels is that they are more or less stable all the way through their production and across their position within the word. We have seen in Week 3 that Spanish vowels are not reduced in unstressed position either.
Some problems might relate to the visual interference in case of cognate words (see Week 2). In these cases the pronunciation of the English vowels can be carried over into Spanish. For example, mission [ˈmɪʃ(ə)n] vs. mission [mi 'sjon]. Note that all the vowels are pronounced differently.
For higher levels: In some varieties of south-eastern Spain (mostly spoken in and close to the province of Granada), most vowels have a more open allophone as well. The difference between the two allophones is significant in the case of the mid vowels /e/ and /o/. The phenomenon is called desdoblamiento vocálico and was first described by Navarro Tomás (1939). In these varieties, word-final /s/ is generally deleted, however, before these silent s’s the open allophones appear and thus the morphological meaning of /s/ (e.g. plural, or verb ending) is not lost; coche ['kot∫e] (car) vs. coches ['kot∫ɛ] (cars).
Ideas for exercises
You might find the recordings on The mimic methodwebpage useful for practising the pronunciation of vowels.
https://www.mimicmethod.com/spanish-pronunciation-ultimate-guide/
References
Hualde, José Ignacio. The Sounds of Spanish, 2005, Cambridge: Cambridge University Press.
Ladefoged, Peter and Johnson, Keith. A Course in Phonetics. (6th ed.), 2010, Boston, Massachusetts: Wadsworth Publishing.
Maddieson, Ian. Patterns of Sounds,1984, Cambridge University Press, Cambridge. Paperback reprint 2009.
Navarro Tomás, Tomás. “Desdoblamiento de fonemas vocálicos”, 1939, Revista de Filología HispánicaVol. 1: 165-7.
Roach, Peter, "British English: Received Pronunciation”, 2004, Journal of the International Phonetic AssociationVol. 34(2): 239-245.