Let's go through each sentence, choose the correct form, and provide translations:
Is/Are there usually an upper limit on the number of managers in a company?
- Correct form: Is
- Translation: Обычно существует верхний предел на количество менеджеров в компании?
As far as I know there is/are no limit.
- Correct form: is
- Translation: Насколько я знаю, предела нет.
There was/were three managers in the company.
- Correct form: were
- Translation: В компании было три менеджера.
There wasn't/weren't many managers in the corporation.
- Correct form: weren't
- Translation: В корпорации не было много менеджеров.
Do you know if there is/are any sales managers in the department?
- Correct form: are
- Translation: Вы знаете, есть ли в отделе какие-либо менеджеры по продажам?
If I am/am not mistaken there is/are not a single sales representative in this area.
- Correct form: am not, is
- Translation: Если я не ошибаюсь, в этом районе нет ни одного торгового представителя.
There is/are a representative in the northern part of the country.
- Correct form: is
- Translation: В северной части страны есть представитель.
In these sentences, choosing between "is" and "are" depends on whether the subject is singular or plural. For instance, "is" is used for singular nouns or non-count nouns, while "are" is used for plural nouns. Similarly, "was" is singular past tense, and "were" is plural past tense.