Qual a finalidade do "do" nas frases seguintes?...

Gostaria de saber o porquê do uso do "do/does" nas frases seguintes. Servem para indicar o tempo verbal presente? As palavras " Seldom", "rarely" e " Not only" fazem parte de alguma classificação que necessite o uso do do/does?

1. Seldom do I study spanish. I don't have free time at all.
2. Rarely does she visit her grandmother.
3. Not only do I teach English, but I also work as a sales manager.

Obrigada!
MENSAGEM PATROCINADA Aprenda dicas sobre os tempos verbais em inglês! Baixe agora o seu Guia Grátis de Tempos Verbais em Inglês. Ele contém um ótimo resumo para revisar todos os conceitos.

Clique aqui e saiba como baixar!
Avatar do usuário Marcio_Farias 12350 1 22 206
Trata-se da inversão inglesa.

but-not-only-did-that-not-happen-por-que-not-duas-vezes-t56929.html

Usually, we put the expression at the beginning of the sentence to emphasise what we're saying. It makes our sentence sound surprising or striking or unusual. It also sounds quite formal. If you don't want to give this impression, you can put the negative expression later in the sentence in the normal way:
Seldom have I seen such beautiful work.
('Seldom' is at the beginning, so we use inversion. This sentence emphasizes what beautiful work it is.)
I have seldom seen such beautiful work.
('Seldom' is in the normal place, so we don't use inversion. This is a normal sentence with no special emphasis.)
Here are some negative adverbs and adverb phrases that we often use with inversion:
Hardly Hardly had I got into bed when the telephone rang.
Never Never had she seen such a beautiful sight before.
Seldom Seldom do we see such an amazing display of dance.
Rarely Rarely will you hear such beautiful music.
Only then Only then did I understand why the tragedy had happened.
Not only ... But Not only does he love chocolate and sweets but he also smokes.
No sooner No sooner had we arrived home than the police rang the doorbell.
Scarcely Scarcely had I got off the bus when it crashed into the back of a car.
Only later Only later did she really think about the situation.
Nowhere Nowhere have I ever had such bad service.
Little Little did he know!
Only in this way Only in this way could John earn enough money to survive.
In no way In no way do I agree with what you're saying.
On no account On no account should you do anything without asking me first.

Fonte: http://www.perfect-english-grammar.com/inversion.html