Learn French: Les flashs éblouissent tellement la star qu'elle tombe. - The flashes dazzle the star so much that she falls.

In French, adverbs that modify a verb usually go immediately after the conjugated verb. Here, tellement modifies the verb éblouissent. The standard pattern is [verb] + tellement + [object] + que + [consequence].

Could I use tant instead of tellement in this sentence?

Yes! When modifying a verb to express intensity leading to a consequence, tellement and tant mean the same thing and are interchangeable. You could perfectly say ...éblouissent tant la star qu'elle tombe.

Why does the verb éblouissent have a double S in the middle?

The infinitive is éblouir (to dazzle), which is a regular -ir verb. Remember that regular -ir verbs add an -iss- to their stem in the plural forms. Since les flashs is plural, we use the ils form, giving us éblouissent.

If the star were a famous male actor, would the ending change to qu'il tombe?

No, it would stay exactly the same! The pronoun elle refers grammatically to the noun la star. Because the word star is always feminine in French regardless of the celebrity's actual gender, any pronoun replacing it must also be feminine.