Use a Paradigm
The next bit of reassurance should be to tell them that just as with the present tense, each person and number has a unique ending. Once again, for regular verbs, they will see the theme vowel in the endings.
Draw the six-hole paradigm on the board to the students' left, labeling it as present tense. Above it, write the infinitive hablar. Next, elicit the conjugation of hablar in the present. This can be done a couple of ways. You can call out, at random and in English, short phrases like he speaks, they speak, I speak, etc., and point to someone to give you the Spanish form, filling in the paradigm as you proceed.
Next, draw another paradigm, labeling it as preterite, and writing the verb hablar above the paradigm. Just before revealing the endings, and as a last-minute review, tell them you are going to ask them the same questions but using spoke instead of speak or speaks. Since they do not know the forms in Spanish at this point, they have to say "first-person singular" if you say I spoke -- and as they do so, you write the Spanish in the appropriate box on the paradigm.