Use the present for current actions, habitual routines, general truths, and with expressions like "hace + time" to describe how long something has been going on.

Signal words: ahorahoysiempretodos los díasnormalmentehace + timeen general
Pronoun Present
yo simulo
simulas
él/ella/Ud. simula
nosotros simulamos
vosotros simuláis
ellos/ellas/Uds. simulan

Use the preterite for actions completed at a specific point in the past — things that started and finished, happened a specific number of times, or occurred in a defined time period.

Signal words: ayeranteayerel lunesla semana pasadauna vezen 2020hace + time + que
Pronoun Preterite
yo simulé
simulaste
él/ella/Ud. simuló
nosotros simulamos
vosotros simulasteis
ellos/ellas/Uds. simularon
Key contrast

"Hablo español" (present — I speak Spanish, habitually) vs "Hablé con él ayer" (preterite — I spoke with him yesterday, completed). The time expression usually makes the choice obvious.

Present vs Preterite: common questions

When should I use the present vs the preterite in Spanish?

Use the present for current or habitual situations, and the preterite for completed past actions. If you can ask "when did it end?" and give a specific answer, use preterite.

How does "hace" work with the present and preterite?

With the present: "Hace dos años que estudio español" = I have been studying Spanish for two years (still ongoing). With the preterite: "Hace dos años estudié en Madrid" = Two years ago I studied in Madrid (completed).

Are there verbs that are almost always in the present tense?

Some verbs like ser, estar, tener, and querer appear very frequently in the present because they describe states rather than discrete events. But all can appear in the preterite when describing a completed state.

Practise both tenses with simular using spaced repetition.