Take understanding further.

Forming the present subjunctive

Take the yo form, drop the -o, then add the opposite-vowel endings: -ar verbs take -e endings, -er/-ir verbs take -a endings.

Spanish · Subjunctive

Preterite vs. imperfect

Preterite = a finished event. Imperfect = an ongoing background or habit.

Spanish · Past

Ser vs. estar

Ser = essence (DOCTOR). Estar = state (PLACE).

Spanish · Copula

Por vs. para

Para points forward (purpose, destination). Por looks backward (cause, exchange, duration).

Spanish · Prepositions

Passé composé vs. imparfait

Passé composé = specific completed event. Imparfait = ongoing background or habit.

French · Past

Forming the present subjunctive

Take the ils/elles present form, drop -ent, add the subjunctive endings.

French · Subjunctive

Choosing être vs. avoir in compound tenses

Most verbs use avoir. A short list of movement/state verbs (and all reflexives) use être.

French · Compound tenses

Forming the conditional

Future stem + imperfect endings.

French · Conditional

The personal infinitive

Portuguese conjugates the infinitive itself when the subject is specified.

Portuguese · Infinitive

Pretérito perfeito vs. imperfeito

Perfeito = completed action. Imperfeito = ongoing or habitual past.

Portuguese · Past

Future subjunctive

Take the 3rd person plural preterite, drop -am, and conjugate.

Portuguese · Subjunctive

Ser vs. estar

Ser = permanent essence. Estar = temporary state or location.

Portuguese · Copula