Bonjour !!!
3- sequence of tenses is what we call in French "concordance des temps". It means to know which tense agrees in a dependant clause according to the tense in the main clause.. or from active voice to passive voice... or from direct speech to indirect speech ..
12- Je voudrais que (conditionnel) + subjunctive present
Je voudrais que tu marches à l'école avec moi
13- Qu'elle vienne ! subjunctive present
This is an exclamative sentence using the subjunctive. It expresses a wish, a will, an order, an injunction ► "je voudrais/ je veux/ j'ordonne qu'elle vienne."
I want/wish/order/ her to come.
Tis is a little different from the imperative "Viens". It's more formal what we call in French "Langage soutenu"
the exact intensity of this subjunctive sentence depends on the context
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14- Il a peur que je ne comprenne ►il a peur (fear) + subjunctive half negative
"il a peur que" or "il craint que" + subjunctive half negative. This is a very formal way to speak. "Langage soutenu" in French.
It means = He's afraid I do understand .
if the main clause is affirmative, then the dependant clause with "ne" also is affirmative... if "pas" isn't written
It's called the "ne" explétif . It's formal and optionnal...
Il a peur que je ne comprenne pas = he is afraid I don't undestand
il a peur que je (ne) comprenne = he is afraid I understand
"ne" alone .. isn't a negation if "pas" doesn't follow
Hope my explanations are clear enough... English isn't my native language !
hope this helps ☺☺☺