I´m a native Spanish speaker and I´m reading the book "Método para aprender latín" by Hermann Schnitzler. Right now, I am doing the exercises of the lesson VI (gender of words from the third declension). In the subsection 5. it ask me to translate to Spanish the following sentence "Vírgines téneræ ab ímprobis príncipum fíliis minis terrentur". The problem is that, at the moment, the book has not taught me the word "téneræ". I just find in Internet as a verb but, I have just learned sentences with only one verb. I share with you the context about the book to see if anybody can share me resources to improve my learning.
This is not the verb tĕnērĕ but the adjective tĕnĕr. In fact, the verb has no form looking like tenerae. What you have is the feminine plural nominative tĕnĕrae, an attribute of vĭrgĭnēs. The subject of the clause you are to translate is thus "tender maids" or something along those lines.
The adjective behaves as if it was tĕnĕrŭs, but the masculine singular nominative (and vocative) lack the ending.
The two words do have forms that look similar. The adverb corresponding to tĕnĕr is tĕnĕrē, and is not to be confused with the infinitive tĕnērĕ. If you mark stress instead of vowel length, the adverb is ténere and the verb is tenére.
Pay attention to vowel length (=quantity). I have indicated here short vowels by a breve (ă) and long ones by a macron (ā). The material you use does not use these, but marks stress with an acute (á) instead. If your material does not indicate vowel length, I recommend finding supporting material that does.