Prepositions and time: FOR vs. SINCE

Use for to introduce durations like 1 second, 3.8 billion years, a long time and a little while.

Life on Earth has existed for about 3.8 billion years.
I listened to a Lady Gaga song for a little while.

Since is used to introduce the point in time when something became true and must always be used with a form of the perfective have.

I have lived on Earth since 1975.
I have been listening to Lady Gaga since 3 o'clock.

Some common mistakes involving since:

WRONG: I have lived here since 30 years. [You can't use a duration with since]
WRONG: I am living here since 1981. [You must use since with a form of have]

See the tag prepositions for related posts.

A conversation at noon

Tenses and aspect are among the most difficult things to learn in any language, so I've made a pretty chart to simplify the whole thing. It includes an example of a conversation that includes each form at least once.

Click to enlarge!

UPDATE:
It helps if you look at which time phrases you can use with each form (left side of the chart). For example:

The race started at 10am. [the simple past]
The race has started now. [has is present tense, so it is about the past of the present (i.e., a time before now)]
The race had started at 11am. [had is past tense, so it is about the past of the past (i.e., a time before 11am)]

If it is now 12pm, you can't say:

WRONG: The race has started at 10am. [has is present tense, but 10am is in the past, so it doesn't make sense]

If you want to attach a time phrase to start instead of have, use the simple past form.

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It's a tag question, innit?

Tag questions are a kind of yes-no question used when you want to confirm that something is true. They are formed by attaching a question tag to the end of a declarative sentence. Here is one example:

He is in jail, is he?

Notice that the tag "is he" has the same form as the beginning of a normal yes-no question that you would use if you didn't have any idea about the answer:

Is he in jail?

I DO TOO and I DON'T EITHER

Use too with a positive sentence:

A: I like chocolate.
B: I like chocolate too!

Use either instead of too when the verb is negated:

A: I don't like chocolate.
B: I don't like chocolate either!

A: I've never liked chocolate.
B: I've never liked chocolate either!

Notice that the form of tag questions also depends on verbal negation:

I like chocolate, don't I?
I don't like chocolate, do I?

The most common words in English

The most common word in English is the. It occurs so frequently that it alone accounts for almost 4% of all words that appear in speech and an amazing 6.4% of words that appear in writing. This means that by learning just this one word, a student of English will already be able to understand 4% of the English words they are ever likely to hear in conversation and 6.4% of the words they read. Better still, a student only needs to learn a very small number of words to increase this percentage dramatically. The nine most common words account for over 20% of speech, and to get to an amazing 50% comprehension of words in spoken English, a student need only learn the 63 most common words. The trick is to know which words these are.

Read on to see a list ordered by frequency.

Uses of the simple present tense

Dynamic verbs
With dynamic verbs (i.e., verbs that describe actions and events), the simple present tense normally has a habitual meaning so can be used with adverbials such as every day and often.

The Sun rises in the east (every day).

To talk about an event that is happening right now, you need to use the progressive -ing form of the verb instead.

The Sun is rising (right now).

Stative verbs
With stative verbs (i.e., verbs that describe states and relations), the simple present tense can't be used with a habitual meaning. Instead, it indicates a state or relation that holds in the present, much as the progressive -ing form is used to indicate the present with dynamic verbs.

I like/love/want/need you.
I come from Berlin.

Stative verbs do not always have a progressive -ing form. When they do, it often forces a dynamic meaning.

WRONG: I am coming from Berlin. [come from in the sense of where a person grew up]
RIGHT: I am coming home (right now). [come in the sense of moving towards]

The place a person comes from is not something that can change, but for states that are changeable such as where a person lives, an -ing form can sometimes be used, particularly when the state is new or likely to change soon:

I live in Berlin. [no comment about how permanent this is]
I am living in Berlin. [suggests that the speaker has moved to Berlin recently or that they might not stay there]

Some other examples of stative verbs are believe, know, agree, appear, seem, contain, include, consist, exist, belong and cost.

Prepositions and time: BY vs. UNTIL

[For German speakers]
The German word bis can be translated into English as either by or until, which makes it hard to know which to use. Here is the answer.

Short version
Use by when talking about an event that occurs at a single point in time like finishing.

I will finish the project by Friday

Use until when talking about an activity or state that continues over a period of time like working (an activity) or being asleep (a state).

I will work until 10pm
I was asleep until the phone rang

Prepositions and places: IN, AT, ON

The U.S. president lives in The White House at number 1600 on Pennsylvania Avenue in the city of Washington in the District of Columbia in the United States on Earth. If these are all places, why does English need three different prepositions (in, at and on) to introduce them? Fortunately, the answer follows from general rules about when each of these prepositions should be used.