My common grammar mistakes + vocabulary link

Comma use:
"Jack who was helping me to...."
Correction:
"Jack, who was helping me to..."

Use a comma to set off parenthetical elements, as in "The Founders Bridge, which spans the Connecticut River, is falling down." By "parenthetical element," we mean a part of a sentence that can be removed without changing the essential meaning of that sentence. The parenthetical element is sometimes called "added information." This is the most difficult rule in punctuation because it is sometimes unclear what is "added" or "parenthetical" and what is essential to the meaning of a sentence.

Example:

Jack, that is from scotland, eats food.

Francisco, who eats people, ate his sister.

http://grammar.ccc.commnet.edu/grammar/commas.htm
Comma use practice: http://www.grammar-monster.com/tests/test_commas.htm ; http://www.bbc.co.uk/skillswise/game/en29punc-game-going-to-work-with-commas

This and These
This (singular)
These (plural)
Example:
This is my house
These are my houses.
Mistake:
This are my plants
Correction:
These are my plants.
http://learnenglish.britishcouncil.org/en/english-grammar/pronouns/that-these-and-those

This and these practice: http://learnenglishkids.britishcouncil.org/en/grammar-games/these-those ; http://www.grammar.cl/Games/This_That_These_Those.htm

PAST SIMPLE:
Something that has been
Regular verbs  = verb + ed
Irregular verbs = depends on the verb

Example:
I walked in the school.
I ran in the school.
http://www.ego4u.com/en/cram-up/grammar/simple-past
Past Simple practice;http://www.eslgamesplus.com/past-simple-irregular-verbs-esl-grammar-interactive-activity-online/ ; http://www.englishclub.com/esl-games/grammar/jumbled-tenses-past-simple-1.htm

Expressing obligation:
have to + infinitive (strong obligation, possibly from outside)
must + infinitive (strong obligation, posibly from the speaker)
Example:
I have to pay my bills.
I must not break the law.
http://www.perfect-english-grammar.com/modal-verbs-of-obligation.html
Practice link; http://www.englishgrammarsecrets.com/musthaveto/exercise1.html ; http://www.englishgrammarsecrets.com/musthaveto/exercise2.html

Expressing purpose:
Use to , so as to, and in order to to express purpose in the affirmative form. 
Use so as not to and in order not to to express purpose in the negative form..
Example:
She told us to do our homework so that we could play.
He taught the kid so as he wouldn't fail his exam.
http://www.myenglishpages.com/site_php_files/grammar-lesson-purpose.php

Practice link: http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-85663.php ;



Vocabulary link:
http://trinity-ise-1-vocabulary.wikia.com/wiki/Health_and_Fitness