Исправьте ошибки: He won't be able run quickly. I am sure you will able to speak Spanish in three month. You don't have to say that. It's not polite. You mustn't wash up. I've already done it. She had to invite Susan's sister to her party, hadn't she?

1). He won't be able to run quickly.

The participle "to" must go after the fixed phrase before the infinitive.

2). I am sure you will be able to speak Spanish in three months.

The fixed phrase is "to be able to". So in the future tense it requires "be" after "will". "Three months" needs the plural form on the noun.

3). You mustn't say that. It's not polite.

The modal "must" expresses the strict prohibition to say the impolite words.

4). You don't need to wash up. I've already done it.

The negative modal "don't need to" shows that is it not obligatory to do something, like "no need".

5). She had to invite Susan's sister to her party, didn't she?

This question requires the tag "didn't she", because the main sentence is in the Past Simple. To make the negative form, we use the auxiliary verb "did" and the participle "not".