"My brother must write that letter."
Translation:Mon frère doit écrire cette lettre.
Yes "falloir" is a defective verb, ie only a few forms actually exist and are used.
On top of the infinitive "falloir" only those are used
- il faut (present)
- il fallait (imparfait/preterit)
- il faudra (future)
- il fallut (passé simple/simple past -> never used on Duolingo)
- il a fallu (passé composé + all the other composed forms: avait fallu, aura fallu, eut fallu, aurait fallu, eût fallu...)
- il faudrait (conditionnel - présent)
- qu'il faille (subjunctive - present)
- qu'il fallût (subjunctive - imparfait)
"must" and "need" have different degrees of 'obligation'.
"he must" is stronger than "he needs" in English and in French as well.
To mean "must" to its full meaning, the French often use "il doit absolument", because "doit", by itself can have several interpretations, depending on context.
In this sentence, though, "he must write" is correctly translated to "il doit écrire".
"ceci" and "cela" are pronouns (= lit. "instead of a noun"), so you cannot have a pronoun in front of a noun.
When this or that are adjectives (in front of a noun), they translate to:
- masc sing in front of a consonant = ce livre
- masc sing in front of a vowel sound = cet arbre, cet homme
- fem sing = cette femme
- masc and fem plural = ces enfants