Remember in school when the teacher assigned a paper, s/he gave a suggested number of pages or words? Remember times you had little to say on that subject but stressed and strained to write "enough." Sometimes, making the margins slightly wider or typing one line short of the bottom line made your paper the "right" length.

In school we learned to write in the passive voice and use progressive verbs to make sentences longer. It takes more words to say the same thing in the passive voice, to use the progressive tense or make convoluted sentences using participles and gerunds. Adding "ing" adds another syllable. The ponderous nature of more words and syllables makes the writing sound weighty and authorative. Don't do it!

Progressive tense

The progressive tense uses the "to be verb" plus an -ing form of the verb. Progressive tenses show a state or action that is continuing or in progress:
He is hitting the baseball over the fence.

The past progressive uses "was/were" plus an -ing form of the verb:
She was kicking a rock down the street.

The future progressive uses "shall be/will be" plus an -ing form:
He will be painting a picture of the sunset.

The progressive tense is not bad grammar. It can be useful unless overused. Eliminate the progressive to make a simple tense (unless progressive is needed) and usually stronger, tighter, more muscular:
He hit the ball over the fence.
She kicked the rock down the street.
He will paint a picture of the sunset.

Participle/gerund phrases

Other words that are out of fashion are gerunds and participles (verbals). Participles take the base verb and add either an -ing or -ed suffix. The suffix doesn't guarantee it's a participle. Only how it's used can do that. If it's used as a noun (beginning its own phrase), it's called a gerund. If it is used as an adjective (beginning its own phrase) and describes the subject of the sentence, it's a participle. Here are examples:

Cruising down the river on a Sunday afternoon, we dreamed of tennis.
[The words from cruising to the comma describe what we were doing. Any -ing word that describes a noun or pronoun is an adjective hence a participle.]

Cutting into the lunch line got him into trouble with the principal.
[From cutting to line is a phrase that acts as the subject of the sentence. Subjects are nouns, so cutting is a gerund.]

Dangling Participles

One reason participles are out-of-fashion is writers use them incorrectly and cause them to dangle. This is not like dangling your feet off a dock but a misuse. Here's an example:

Dangling their feet in the water, the wake from the motorboat splashed into their laps.
[The dangling phrase is supposed to be describing the subject of the sentence, wake. So the wake was dangling feet? Nope!]

Hitting her with a baseball bat, Sally's arm was broken.
[First, this is in the passive voice. Probably not a good start. The phrase beginning with hitting should describe the subject arm, but doesn't.]

To test a questionable sentence, put the phrase after the subject to see if it makes sense. [The wake, dangling their feet . . . . Sally's arm, hitting her with . . . .

Can they do that?

Verbs tell time, and one phrase may tell that something happened in one time frame and another phrase another time frame. Sometimes you want to tell the reader that one thing happened while something else was happening or two things happened simultaneously. All this slight-of-hand is done with verbs and to some extent verbals. But when the writer gets it wrong, the reader either laughs or stops reading or both.

Ducking into the tent, she was changing her jeans.
[So, she was taking her pants off WHILE she was ducking? Embarrassing. Even Ducking into the tent, she changed her jeans doesn't work.]

Heading for the street, he left the playground.
[So he was heading for WHILE he was leaving the playground.]

The Real Problem

-Ing words are grammatical and respectable, but some writers love them and use them everywhere. They scatter the page with gerunds, participle and the progressive tense. The writer feels his words are more present and now. But don't be like some who are so terrified of -ing they avoid them altogether to the point of using bad constructions to avoid their use. The answer lies in moderation and knowing what each verb tense does. Yes, you need a variety of sentences. Use simple, compound and complex unless you write easy readers or early chapter books. And use -ing words in moderation because they are out-of-fashion.